The pokemon must be OU or lower. You can use as many coutners as you want (but a balanced team with an extra counter or two is probably best)
The gym leader only has to use 3 of their type pokemons, the rest can be any Ou or less pokes.
You only have to win one game. The gym leader can allow rematches right away if they want. Win four badges and you can be a gym leader.
I have to go to church soon, sorry