How many unique Pokémon formes are there?

Question

Some Pokémon only have one version and one shiny version, so 2 total formes/sprites/call them whatever you like. Some others, however, have several. Examples include Vivillon (20 known patterns), Meowstic (prominent gender differences) and Deoxys (different activatable formes).

I'm looking for the total number excluding Mega Evolutions, Spiky-Eared Pichu and Spinda, specifically:

• How many unique formes are there at all?
• How many unique formes can one have at the same time, assuming enough Pokémon are accessible? (As far as I know, you can't have both Black and White Kyurem at the same time, for example)
• How many of these are available in storage, that is, don't get lost when the Pokémon is put into a PC box?

It would be great to have the answer updated as new Pokémon and formes are released, but in any case, I'd like to have an answer as of X/Y.

The versions count as different if they are visually different. Things such as evolution that differs depending on gender don't count as different formes.

Answer 1

As of Sun & Moon, the post below is no longer accurate. I will leave it below unedited for reference purposes.

New in Sun & Moon:

• 81 new Pokemon with their own National Pokedex number
• 18 new regional variants (Alolan Forms) of Gen I Pokemon (Rattata, Raticate, Raichu, Sandshrew, Sandslash, Vulpix, Ninetales, Diglett, Dugtrio, Meowth, Persian, Geodude, Graveler, Golem, Grimer, Muk, Exeggutor, Marowak)
• 11 new additional storable Formes (1 Zygarde, 3 Oricorio, 1 Lycanroc, 6 Minior)
• 17 new additional Item-induced Formes (Silvally)
• 6 new Battle-activated Formes (1 Zygarde, 1 Wishiwashi, 1 Minior, 1 Mimikyu, 1 Solgaleo, 1 Lunala)
• All of these in a shiny version, of which 18 are unobtainable (4 Tapus, Cosmog, Cosmoem, Solgaleo, Lunala, 7 Ultra Beasts, Necrozma, Magearna, Marshadow)

This puts the new totals at 1990 unique storable sprites (1789+(81+18+10)*2-18 +1 Zygarde), 2018 including unobtainable ones and 2133 total sprites (1889+(81+18+10)*2-18+(17+5)*2 -1 Solgaleo -1 Lunala +2 Zygarde).

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After quite some research time and thanks to Jerry's help, I was able to compile a complete list. Here are the numbers involved. As per the question, Mega Evolutions are excluded. Also excluded is Spiky-Eared Pichu, which only existed in Gen IV and cannot be transferred to newer generations, as well as Spinda, which has an absurd amount of technically different sprites (one per 32-bit PID).

• There are currently 721 Pokémon on the National Pokédex, including Diancie, Hoopa and Volcanion.
• Of these, 91 females have minor gender differences and 7 females have major differences (Hippopotas, Hippowdon, Unfezant, Frillish, Jellicent, Pyroar and Meowstick).
• The number of additional storable (permanent) alternate formes is 81 (27 Unown, 3 Deoxys, 2 Burmy, 2 Wormadam, 1 Shellos, 1 Gastrodon, 1 Basculin, 3 Deerling, 3 Sawsbuck, 1 Keldeo, 19 Vivillon, 4 Flabébé, 4 Floette, 4 Florges, 3 Pumpkaboo, 3 Gourgeist).
• There are 34 additional Item-activated alternate formes (5 Rotom, 1 Giratina, 1 Shaymin, 17 Arceus, 1 Tornadus, 1 Thundurus, 1 Landorus, 2 Kyurem, 4 Genesect, 1 Hoopa).
• You can transform Pokémon into 7 additional Battle-based formes (3 Castform, 1 Cherrim, 1 Darmanitan, 1 Meloetta, 1 Aegislash).
• Then there are 10 additional special formes (9 Furfrou, 1 Xerneas).
• And lastly, all of these exist in a shiny version, except for 8 non-obtainable shinies and 3 alternate formes (Celebi, Victini, Keldeo, Resolute Keldeo, Meloetta, Poké Ball Vivillon, Fancy Vivillon, Zygarde, Diancie, Hoopa, Volcanion).

This means that the total number of storable unique sprites is 1789 ( (721+91+7+81)*2 -8 -1 Keldeo -2 Vivillon, interestingly exactly 1800 including currently unobtainable ones).

The total number of sprites including battle-induced, item-induced and special forms is 1889 (1789 + (34+7+10)*2 -1 Meloetta -1 Hoopa).

Answer 2

Bulbapedia has a list of Pokémon forms. As of this time, it contains:

• Pichu, which can be Spiky-Eared Pichu
• Unown, which have a total of 28 different forms (the 26 letters of the alphabet plus ? and !)
• Castform, which due to its ability Forecast, can turn into Sunny, Rainy, and Snowy forms based on the weather
• Deoxys, which can be in Normal, Attack, Defense, or Speed forms based on game (in Gen III) or based on what meteorite you just inspected (Gens IV, V, VI)
• Burmy and female Wormadam, which can have Plant, Sandy, or Trash cloaks based on what terrain it last battled in (as a Burmy)
• Cherrim, which due to its ability Flower Gift, flips between Overcast and Sunshine forms based on whether the weather is sunny
• Shellos and Gastorodon, which have different appearances depending on whether they come from the West coast or the East coast
• Rotom, which can possess appliances to convert to Heat, Wash, Frost, Fan, or Mow forms
• Giratina, which is in its Origin form inside the Distortion World (or it holds the Griseous Orb) and its Altered form otherwise
• Shaymin, which turns into its Sky form upon contact with the Gracidea item and reverts to its Land form at night or if frozen
• Basculin, which can be Red-Striped or Blue-Striped depending whether the game is Black(2) or White(2) (though rare instances of the other can occur)
• Darmanitan, which flips between Standard and Zen mode based on whether its HP is above or below half
• Deerling and Sawsbuck, which match the season of Spring, Summer, Autumn, and Winter (anything other than Spring has to be transferred in Gen VI, as seasons do not exist there)
• Tornadus, Thundurus, and Landorus, which depending on how they are obtained can be in their Incarnate or Therian form
• Kyruem, which can be fused/unfused to Reshiram or Zekrom with DNA Splicers to form White or Black forms
• Keldeo, which converts between Ordinary and Resolute forms depending on whether it knows Secret Sword
• Meloetta, which temporarily changes from Aria to Pirouette form if it uses Relic Song
• Vivillion, which has 18 different player-location-based forms (Archipelago, Continental, Elegant, Garden, High Plains, Icy Snow, Jungle, Marine, Meadow, Modern, Monsoon, Ocean, Polar, River, Sandstorm, Savanna, Sun, Tundra) and 2 event forms (Poké Ball, Fancy)
• Flabébé, Floette, and Florges, which match the colour of the flower bed they're caught in (Red, Yellow, Orange, Blue, White)
• Furfrou, which can be groomed from Normal to Heart, Star, Diamond, Debutante, Matron, Dandy, La Reine, Kabuki, or Pharaoh trims for 5 days
• Aegislash, which converts between Blade and Shield forms when it uses offensive moves/King's Shield
• Pumpkaboo and Gourgeist, which come in Small, Average, Large, and Super sizes
• Xerneas, which is in Neutral mode for cutscenes and Active mdoe for battle