What is a good winrate on your main

I'd say 55% or 60%

60 percent other wise you simply aren't climbing at a good rate.

50/55 means you are breaking even and relying on luck to win you the other 50 percent.

60 percent means you are good enough to rely on yourself, and only need luck to win you the final 40 percent.

55% is good when im climbing

really anything over 50 is good because it means you are gaining elo and rank

Is there a certain amount of games the winrate should matter? like 50 games, 100 games?

anything above 54% usually but it depends on number of games. a 54% win rate with a total of 20 games played is kinda shitty. however a 54% win rate with 200 games is pretty good.

Riot says 50% winrate on Mordekaiser is good. Any higher and he becomes Troublesome

Depends on what your main is, 30% is probably fine on aatrox