That annoys me as well, especially in Hoenn which I felt they could have did a lot more considering the fresh start he had. My thought is they have at least three companions so like you said give him 5-8. Then they give the two companions about the same so they can showcase several lines and not just have Ash as the main showcaser of the pokemon. Still I think he should catch more, but that is just me wanting to see pokemon I like in his stead.
Bulba refused, Squirtle I dunno why maybe they just never got around to it or felt it better to use him going to the Squirtle Squad remaining a Squirtle instead of a Wartotle Squad or even Blastoise. His Johto team I felt were not adequately trained as he tended to have Kanto pokemon relied on for an at least partial duration of the journey in Johto. Hoenn I thought they could have at least got a Crawdaunt out of it. Sinnoh I felt a step in the right direction he had two fully evolved starters for the first time. Unova he will end up with at least 3 fully evolved pokemon. Which I could easily see it as 5-6 depending on how they handle Palitoad and Boldore who could evolve at any time. And maybe they get Emboar as well.
Well the series will probably run for a long time so only time will tell. I sure who the writers have something good in store for him. But if they do I don't want it to be an overly relied poke. Just something to counter balance trainers like Trollbias.