I went with 0, because for one part, if you have a lengendary, then why not have more, you are already tipping the scales in your favor, so go nuts I guess.
But then there are also other factors, like how good legendaries actually are. For instance, based on Smogon's tier ranking, some legendaries are actually pretty bad, like to the extent that they aren't even in the upper tiers. For instance, Entei is in the Rarely-Used tier(The other tiers being Never-Used, Under-Used, Overused, and Uber if you aren't familiar). And then Regigigas is in the Never-Used tier, due to it's horribly prohibitive ability.
So with some cases like legendaries being shitty, or regular pokemon being horrendously OP, there can be some wiggle room in how many legendaries should be allowed on a team.
But in general, if you want to have a power limit, then you should take actual stats/abilities/utilities into consideration instead of just banning legendaries, because not all legendaries are created equal, and some regular pokemon are just as capable as OP legendaries.