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the number of pokemon in the gym depends on the gym level. you can level up a gym of your own team by 'training' which is basically a battle but you can only use one of your pokemon instead of 6. i've found it's much easier to install a pokemon by taking over a rival gym rather than propping up an existing gym, however if you do the latter (and your pokemon is top) it will last in the gym longer.
I've tried all of those things but sometimes I fight only one guy and other times I get a long procession of Pokémon and the fight times out before I can seal the deal... Usually with me in a winning position, which is bloody annoying...