Well, in Kalos, most of his Pokemon were fully evolved, that's why he had less Pokemon. But his team was really strong. But bringing 3 Pokemon to Oak's lab I think was really poor. Anyway, we don't know that if there will be a League in Alola having the 6 on 6 battle format. So I'm not too sure about Ash catching too many Pokemon in SM.
Fully evolved=/= less Pokemon.He could've easily captured Trevenant and Avalugg to justify Goodra's departure( talking prior the league)
Or just Avalugg or Trevenant. Why? Remember Fletchinder-Moltres and Noibat-Zapdos? Where is Articuno? Like Bergmite-Articuno? SMH. That's why Ash needs to catch 6-7 Alolan Pokemon to satisfy himself from Kalos.
Every league has 6 vs 6 format(some of them are like Kalos, some like Hoenn). Even if this league only has champion vs ??? ( like Orange Islands)Ash still needs 6 Pokemon.
I think this league will justify Unova league. So, 128 people: 1 vs 1
64-16: 3 vs 3( 2 vs 2 in Unova were do forced,except Top 16)
Top 8 -final: 6 vs 6
IMO:
/-replacements, *-potential but unlikely.
Ash's the strongest Pokemon in Alola:
Decidueye=Incineroar>Pikachu>Vikavolt/Golisopod>Lycanroc(Midday/Midnight)>Toucannon*>Hakamo-o>>Crabrawler/Passimian