Wait, didn't he have a bunch of Oak warmers like Tauros, Muk and Kingler?
iirc his team that maxxed out first was Pikachu, 3 starters, Pidgeotto and Butterfree. Then he caught Krabby, Muk and Tauros later on after Butterfree left, with the Krabby catch being a plotpoint about being his 7th catch.
While Ash did catch Tauros, Muk and Krabby they never officially joined his team and instead were shipped off to Oak, they never join the team when Ash's 6th slot was free.
Plus Tauros and Muk were caught as a gag joke with Tauros popping up to ruining Ash catching a Safari Pokémon and Muk being so smelly that it could be smelt from the Pokéball.
They didn't officially became active member till much later, Muk being used in the Kanto league and Tauros during the Orange Island league because Ash couldn't wake Snorlax up and needed to swap him out.
And speaking of which, I realized what a terrible premise that episode started with. "Ash didn't catch a Pokemon on his own". WELL DUH, TRAINERS CATCH POKEMON WITH THEIR POKEMON. What the fuck.
The point they were trying to make was that Charmander, Bulbasuar and Squirtle joined Ash's team out of their own free will.
Though the point is moot since,
A. Ash caught Pidgeotto on his own. B he battled Bulbasuar first before he joined the team and C He caught Catapie on his own without any Pokémon.
Then again Misty was a jerk and this lead to Ash catching his one time deus ex machina Krabby.