Reigonal Dex specifically:
Red, Blue, Yellow have only Poliwag, Poliwhirl, Poliwrath, Bulbasaur, Ivysaur, and Venusaur, so 6.
Gold, Silver, and, Crystal have Poliwag, Poliwhirl, Poliwrath, and Politoed, so 4.
Ruby, Sapphire, and Emerald had the entire Poliwag line, but you have to trade to get them in Ruby and Sapphire, and you can't get them before the national dex in Emerald. So technically 4, but not really.
FireRed and LeafGreen have Poliwag, Poliwhirl, Poliwrath, Politoed, Bulbasaur, Ivysaur, Venusaur, so 7.
Diamond, Pearl, and Platinum have Croagunk, Toxicroak, Poliwag, Poliwhirl, Poliwrath, and Politoed. So 6.
HeartGold and SoulSilver have the exact same as Diamond, Pearl, Platinum. So 6.
Black, White, Black 2, White 2 have Tympole, Palpitoad, and Seismitoad. They have 3 each.
And then comes X and Y. Bulbasaur, Ivysaur, Venusaur, Froakie, Frogadier, Greninja, Poliwag, Poliwhirl, Poliwrath, Politoed. 10!
Omega Ruby and Alpha Sapphire are robbed of frogs just like their predecessors. 0.
Sun, Moon, Ultra Sun, Ultra Moon have Poliwag, Poliwhirl, Poliwrath, Politoed. That's 4. Plus Bulbasaur, Ivysaur, Venusaur, Froakie, Frogadier, Greninja can be gotten through Island Scan in the Ultra games. So it could 10, but only in the Ultra games.
Let's Go Pikachu and Eevee have Bulbasaur, Ivysaur, Venusaur, Poliwag, Poliwhirl, Poliwrath. 6.
Sword and Shield have Tympole, Palpitoad, and Seismitoad. 3.
But with the DLC, Sword and Shield have Bulbasaur, Ivysaur, Venusaur, Poliwag, Poliwhirl, Poliwrath, Politoed, Croagunk, Toxicroak, Tympole, Palpitoad, Seismitoad. 9.
Brilliant Diamond and Shining Pearl have Poliwag, Poliwhirl, Poliwrath, Politoed, Croagunk, Toxicroak. 6.
Legends: Arceus has Croagunk and Toxicroak. Only 2.
Scarlet and Violet have Croagunk, Toxicroak, Tadbulb, Bellibolt. 4.
But with the DLC, they have Bulbasaur, Ivysaur, Venusaur, Poliwag, Poliwhirl, Poliwrath, Politoed, Froakie, Frogadier, Greninja. So 14 total.
Legends Z-A has Bulbasaur, Ivysaur, Venusaur, Froakie, Frogadier, Greninja. 6
The highest Switch game frog count is... Drumroll, please: Scarlet and Violet w/Teal Mask & Indigo Disk DLC. With 14 total frog Pokemon and 6 fully evolved frogs. But without DLC, it's Z-A, BDSP, and Let's Go.
The highest total frog count game before the Switch, is X and Y or Ultra Sun & Ultra Moon, with 10 frogs, and 4 fully evolved ones.
