The number of petals a flower has can be an important key to identifying the flower. Newcombe's Wildflower Guide is based on this concept.
An important thing to remember about this concept is that the "number of petals" represents the number of apparent petals. This is not necessarily the botanically-correct number of petals. Oftentimes what appear to be petals are actually sepals or bracts. It is an aid to identification, not a botanical classification.

Agrimony (Agrimonia gryposepala)
Agrimony, Small-flowered (Agrimonia parviflora)
Alexanders, Golden (Zizia aurea)
Anemone, Canada (Anemone canadensis)
Anemone, Long-fruited (Anemone cylindrica)
Anemone, Small-flowered (Anemone parviflora)
Anemone, Wood (Anemone quinquefolia)
Angelica, Great (Angelica atropurpurea)
Aniseroot (Osmorhiza longistylis)
Arbutus, Trailing (Epigaea repens)
Avens, Catlings (Geum X catlingii)
Avens, Spring (Geum vernum)
Avens, Water (Geum rivale)
Avens, White (Geum canadense)
Avens, Wood (Geum urbanum)
Avens, Yellow (Geum aleppicum)
Baked-apple Berry (Rubus chamaemorus)
Bearberry (Arctostaphylos uva-ursi)
Bellflower, Clustered (Campanula glomerata)
Bellflower, Creeping (Campanula rapunculoides)
Bellflower, Marsh (Campanula aparinoides)
Bindweed, Eurasian Black (Fallopia convolvulus)
Bindweed, Field (Convolvulus arvensis)
Bindweed, Hedge (Calystegia sepium)
Bindweed, Upright (Calystegia spithamaea)
Bistort, Alpine (Bistorta vivipara)
Bluebells, Northern (Mertensia paniculata)
Buckbean (Menyanthes trifoliata)
Buttercup, Hispid (Ranunculus hispidus)
Buttercup, White Water (Ranunculus aquatilis)
Campion, Bladder (Silene vulgaris)
Campion, Red (Lychnis dioica)
Campion, White (Silene latifolia)
Cancerroot, One-flowered (Orobanche uniflora)
Carrot, Wild (Daucus carota)
Catchfly, Night-flowering (Silene noctiflora)
Centaury (Centaurium erythraea)
Chickweed, Field (Cerastium arvense)
Chickweed, Mouse-ear (Cerastium fontanum)
Cicely, Sweet (Osmorhiza claytonii)
Cinquefoil, Common (Potentilla simplex)
Cinquefoil, Tall (Drymocallis arguta)
Cinquefoil, Three-toothed (Sibbaldia tridentata)
Columbine, Wild (Aquilegia canadensis)
Cranesbill, Bicknell's (Geranium bicknellii)
Creeper, Trumpet (Campsis radicans)
Crowfoot, Bristly (Ranunculus pensylvanicus)
Crowfoot, Cursed (Ranunculus sceleratus)
Crowfoot, Hooked (Ranunculus recurvatus)
Crowfoot, Small Yellow Water (Ranunculus gmelinii)
Crowfoot, Small-flowered (Ranunculus abortivus)
Cucumber, One-seeded Bur (Sicyos angulatus)
Foamflower (Tiarella cordifolia)
Geranium, Wild (Geranium maculatum)
Ginseng (Panax quinquefolius)
Ginseng, Dwarf (Panax trifolius)
Goldthread (Coptis trifolia)
Grass-of-Parnassus (Parnassia glauca)
Harbinger-of-Spring (Erigenia bulbosa)
Harebell (Campanula rotundifolia)
Heather, False (Hudsonia tomentosa)
Hedge Parsley, Japanese (Torilis japonica)
Hemlock, Bulb-bearing Water (Cicuta bulbifera)
Hemlock, Poison (Conium maculatum)
Hemlock, Water (Cicuta maculata)
Herb Robert (Geranium robertianum)
Honewort (Cryptotaenia canadensis)
Honeysuckle, Bush (Diervilla lonicera)
Hound's Tongue (Cynoglossum officinale)
Indian Pipe (Monotropa uniflora)
Ironweed, Tall (Vernonia gigantea)
Ivy, Poison (Toxicodendron rydbergii)
Jointweed, Northern (Polygonum articulatum)
Knotweed, Japanese (Fallopia japonica)
Leatherleaf (Chamaedaphne calyculata)
Looking-glass, Venus's (Triodanis perfoliata)
Loosestrife, Fringed (Lysimachia ciliata)
Loosestrife, Prairie (Lysimachia quadriflora)
Loosestrife, Purple (Lythrum salicaria)
Loosestrife, Swamp (Decodon verticillatus)
Loosestrife, Tufted (Lysimachia thyrsiflora)
Loosestrife, Whorled (Lysimachia quadrifolia)
Loosestrife, Yellow (Lysimachia terrestris)
Lungwort, Sea (Mertensia maritima)
Marigold, Marsh (Caltha palustris)
Milkweed, Butterfly (Asclepias tuberosa)
Milkweed, Common (Asclepias syriaca)
Milkweed, Poke (Asclepias exaltata)
Milkweed, Swamp (Asclepias incarnata)
Milkweed, Whorled (Asclepias verticillata)
Miterwort (Mitella diphylla)
Miterwort, Naked (Mitella nuda)
Mullein, Common (Verbascum thapsus)
Mullein, Dark (Verbascum nigrum)
Mullein, Moth (Verbascum blattaria)
Parsnip, Water (Sium suave)
Pennyroyal, False (Trichostema brachiatum)
Periwinkle (Vinca minor)
Pimpernel, Yellow (Taenidia integerrima)
Pinesap (Hypopitys monotropa)
Pipsissewa (Chimaphila umbellata)
Pokeweed (Phytolacca americana)
Primrose, Dwarf Canadian (Primula mistassinica)
Purslane (Portulaca oleracea)
Pyrola, One-Flowered (Moneses uniflora)
Pyrola, One-sided (Orthilia secunda)
Rattlesnakeroot, Tall (Nabalus altissimus)
Sandwort, Rock (Sabulina michauxii)
Sandwort, Thyme-leaved (Arenaria serpyllifolia)
Saxifrage, Early (Micranthes virginiensis)
Scilla (Scilla siberica)
Senna, Wild (Senna hebecarpa)
Shinleaf (Pyrola elliptica)
Silverweed (Potentilla anserina)
Snow in Summer (Cerastium tomentosum)
Sorrel, Common Wood (Oxalis montana)
Spring Beauty, Carolina (Claytonia caroliniana)
Spring Beauty, Narrow-leaved (Claytonia virginica)
Spurge, Flowering (Euphorbia corollata)
Stonecrop, Mossy (Sedum acre)
Strawberry, Barren (Geum fragarioides)
Strawberry, Indian (Potentilla indica)
Strawberry, Wild (Fragaria virginiana)
Strawberry, Woodland (Fragaria vesca)
Sundew, Linear-leaved (Drosera linearis)
Sundew, Round-leaved (Drosera rotundifolia)
Sundew, Spatulate-leaved (Drosera intermedia)
Swallowwort, Black (Cynanchum louiseae)
Swallowwort, Pale (Cynanchum rossicum)
Swallowwort, White (Cynanchum vincetoxicum)
Thimbleweed (Anemone virginiana)
Tulip (Tulipa sylvestris)
Twinflower (Linnaea borealis)
Velvetleaf (Abutilon theophrasti)
Vervain, Blue (Verbena hastata)
Vervain, Hoary (Verbena stricta)
Waterleaf, Virginia (Hydrophyllum virginianum)
Wintergreen (Gaultheria procumbens)
Yarrow (Achillea millefolium)

Irregular 2 petals 3 petals 4 petals 5 petals 6 petals 7 or more Indistinguishable

THUMBNAILS
Agrimony (Agrimonia gryposepala)	Agrimony, Small-flowered (Agrimonia parviflora)	Alexanders, Golden (Zizia aurea)	Anemone, Canada (Anemone canadensis)

Anemone, Long-fruited (Anemone cylindrica)	Anemone, Small-flowered (Anemone parviflora)	Anemone, Wood (Anemone quinquefolia)	Angelica, Great (Angelica atropurpurea)

Aniseroot (Osmorhiza longistylis)	Arbutus, Trailing (Epigaea repens)	Avens, Catlings (Geum X catlingii)	Avens, Spring (Geum vernum)

Avens, Water (Geum rivale)	Avens, White (Geum canadense)	Avens, Wood (Geum urbanum)	Avens, Yellow (Geum aleppicum)

Baked-apple Berry (Rubus chamaemorus)	Bearberry (Arctostaphylos uva-ursi)	Bellflower, Clustered (Campanula glomerata)	Bellflower, Creeping (Campanula rapunculoides)

Bellflower, Marsh (Campanula aparinoides)	Bindweed, Eurasian Black (Fallopia convolvulus)	Bindweed, Field (Convolvulus arvensis)	Bindweed, Hedge (Calystegia sepium)

Bindweed, Upright (Calystegia spithamaea)	Bistort, Alpine (Bistorta vivipara)	Bluebells, Northern (Mertensia paniculata)	Buckbean (Menyanthes trifoliata)

Buttercup, Hispid (Ranunculus hispidus)	Buttercup, White Water (Ranunculus aquatilis)	Campion, Bladder (Silene vulgaris)	Campion, Red (Lychnis dioica)

Campion, White (Silene latifolia)	Cancerroot, One-flowered (Orobanche uniflora)	Catchfly, Night-flowering (Silene noctiflora)	Centaury (Centaurium erythraea)

Chickweed, Field (Cerastium arvense)	Chickweed, Mouse-ear (Cerastium fontanum)	Cicely, Sweet (Osmorhiza claytonii)	Cinquefoil, Three-toothed (Sibbaldia tridentata)

Cranesbill, Bicknell's (Geranium bicknellii)	Crowfoot, Bristly (Ranunculus pensylvanicus)	Crowfoot, Cursed (Ranunculus sceleratus)	Crowfoot, Hooked (Ranunculus recurvatus)

Crowfoot, Small Yellow Water (Ranunculus gmelinii)	Crowfoot, Small-flowered (Ranunculus abortivus)	Cucumber, One-seeded Bur (Sicyos angulatus)	Foamflower (Tiarella cordifolia)

Geranium, Wild (Geranium maculatum)	Ginseng (Panax quinquefolius)	Ginseng, Dwarf (Panax trifolius)	Goldthread (Coptis trifolia)

Grass-of-Parnassus (Parnassia glauca)	Harebell (Campanula rotundifolia)	Heather, False (Hudsonia tomentosa)	Herb Robert (Geranium robertianum)

Hound's Tongue (Cynoglossum officinale)	Indian Pipe (Monotropa uniflora)	Ivy, Poison (Toxicodendron rydbergii)	Knotweed, Japanese (Fallopia japonica)

Leatherleaf (Chamaedaphne calyculata)	Looking-glass, Venus's (Triodanis perfoliata)	Marigold, Marsh (Caltha palustris)	Milkweed, Butterfly (Asclepias tuberosa)

Miterwort (Mitella diphylla)	Miterwort, Naked (Mitella nuda)	Pennyroyal, False (Trichostema brachiatum)	Periwinkle (Vinca minor)

Pimpernel, Yellow (Taenidia integerrima)	Pinesap (Hypopitys monotropa)	Pipsissewa (Chimaphila umbellata)	Pokeweed (Phytolacca americana)

Primrose, Dwarf Canadian (Primula mistassinica)	Purslane (Portulaca oleracea)	Pyrola, One-sided (Orthilia secunda)	Rattlesnakeroot, Tall (Nabalus altissimus)

Sandwort, Rock (Sabulina michauxii)	Sandwort, Thyme-leaved (Arenaria serpyllifolia)	Saxifrage, Early (Micranthes virginiensis)	Scilla (Scilla siberica)

Senna, Wild (Senna hebecarpa)	Shinleaf (Pyrola elliptica)	Silverweed (Potentilla anserina)	Snow in Summer (Cerastium tomentosum)

Sorrel, Common Wood (Oxalis montana)	Spring Beauty, Carolina (Claytonia caroliniana)	Spring Beauty, Narrow-leaved (Claytonia virginica)	Stonecrop, Mossy (Sedum acre)

Strawberry, Barren (Geum fragarioides)	Strawberry, Indian (Potentilla indica)	Strawberry, Wild (Fragaria virginiana)	Strawberry, Woodland (Fragaria vesca)

Sundew, Linear-leaved (Drosera linearis)	Thimbleweed (Anemone virginiana)	Tulip (Tulipa sylvestris)	Twinflower (Linnaea borealis)

Velvetleaf (Abutilon theophrasti)	Waterleaf, Virginia (Hydrophyllum virginianum)	Wintergreen (Gaultheria procumbens)	Yarrow (Achillea millefolium)

Irregular 2 petals 3 petals 4 petals 5 petals 6 petals 7 or more Indistinguishable

To see all Species with 5 Flower Parts in order by Common Name → START HERE