I'm only counting base forms. Otherwise, Arceus and Silvally count for everything, and Rotom counts for quite a few types as well.
Normal:
9 (Arceus, Ditto, Meloetta, Porygon, Porygon-Z, Porygon2, Regigigas, Silvally, Type: Null)
Fire:
7 (Blacephalon, Ho-Oh, Moltres, Reshiram, Victini, Volcanion)
Water:
12 (Arctovish, Dracovish, Keldeo, Kyogre, Manaphy, Palkia, Phione, Starmie, Staryu, Suicune, Tapu Fini, Volcanion)
Electric:
13 (Arctozolt, Dracozolt, Electrode, Magnemite, Magneton, Magnezone, Raikou, Regieleki, Rotom, Tapu Koko, Zapdos, Zekrom, Zeraora)
Grass:
8 (Calyrex, Celebi, Dhelmise, Kartana, Shaymin, Tapu Bulu, Virizion, Zarude)
Ice:
7 (Arctovish, Arctozolt, Articuno, Cryogonal, Kyurem, Regice, Glastrier)
Fighting:
9 (Buzzwole, Cobalion, Falinks, Keldeo, Marshadow, Pheromosa, Terrakion, Virizion, Zamazenta)
Poison:
4 (Eternatus, Naganadel, Nihilego, Poipole)
Ground:
5 (Baltoy, Claydol, Golett, Golurk, Zygarde)
Flying:
9 (Articuno, Celesteela, Ho-Oh, Lugia, Minior, Moltres, Rayquaza, Yveltal, Zapdos)
Psychic:
29 (Azelf, Baltoy, Beldum, Bronzong, Bronzor, Calyrex, Celebi, Claydol, Cosmoem, Cosmog, Deoxys, Hoopa, Jirachi, Lugia, Lunala, Lunatone, Mesprit, Metagross, Metang, Mew, Mewtwo, Necrozma, Solgaleo, Solrock, Starmie, Tapu Lele, Unown, Uxie, Victini)
Bug:
4 (Buzzwole, Genesect, Pheromosa, Shedinja)
Rock:
9 (Carbink, Diancie, Lunatone, Minior, Nihilego, Regirock, Solrock, Stakataka, Terrakion)
Ghost:
12 (Dhelmise, Giratina, Golett, Golurk, Hoopa, Lunala, Marshadow, Polteageist, Rotom, Shedinja, Sinistea, Spectrier)
Dragon:
15 (Dialga, Dracovish, Dracozolt, Eternatus, Giratina, Guzzlord, Kyurem, Naganadel, Palkia, Poipole, Rayquaza, Regidrago, Reshiram, Zekrom, Zygarde)
Dark:
4 (Darkrai, Guzzlord, Yveltal, Zarude)
Steel:
20 (Beldum, Bronzong, Bronzor, Celesteela, Dialga, Genesect, Jirachi, Kartana, Klang, Klink, Klinklang, Magearna, Magnemite, Magneton, Magnezone, Melmetal, Meltan, Metagross, Metang, Registeel)
Fairy:
9 (Carbink, Diancie, Magearna, Tapu Bulu, Tapu Fini, Tapu Koko, Tapu Lele, Xerneas, Zacian)
Psychic has the most, with Steel in second.
Source
Hope I helped!
