Maximum of 4 players from a single club (maximum of 2 players over 23)
I don't think there's no limit to the amount in total but considering you can only select 5 in the matchday squad (or 4 if a emergency keeper is brought in) any more than 5 is a bit of a waste. I guess a club could send a player on loan to earn a spot here.
As for contributions, we can pay the whole of his wage or none of it and anywhere in between. If its a promising youngster we may get him for free on the caveat that we play him from the bench for example.
I know what I'm doing
You're an idiot and I'm poking you with a stick