not including the 4 copies rule
you can have as many different Pokemon ex in your deck as you want, as long as they're different. the 4 copy rule applies to everything that isn't an Energy, Prism Star, Ace Spec or Radiant Pokemon. so the 4 copy rule is the only thing holding how many ex's you use back. but other than that you could have a deck with no Pokemon but Pokemon ex's and it would be fine.
source: I've played the TCG since Sun/Moon and know this stuff and this https://www.reddit.com/r/PokemonTCG/comments/17pi19d/how_many_exexgxvmaxvstar_etc_cards_can_go_in_a/
