Currently I have about 260 friends, but generally I try to keep a limit of 250 friends, always delete the inactive. This way I can always have between 30 and 70 supporters in my matches, if they are not at night or something.
The problem is, if you have more than 300 friends, it will get very messy with the matches and the game secretly removes some of your friends, so that you can't support them and they don't see your matches as well.