Besides his main party, Wily 'has' the 14 other Genesect Units that spend all their time helping people around town and being a tourist attraction.

Most of Isbar's League have a good 6-10 Pokemon ready besides their main team, since they change strategies at least once every league cycle.

The current Champion set up type-specific training facilities open to the public that are overseen by each type-specialist, so it's easy for them to keep their pokemon in fighting shape.

The ones with the most mons are Albright, who basically has a whole ranch/nature preserve, and the Champion who's constantly training new Pokemon and has over 40 Pokemon ready for battle.