100 friends is the most the game will allow you to have.
Your friends get an error message if they try to add you when you’ve already reached the limit – and the same thing happens if you try to add someone who already has 100 friends.
Most likely the limit is due to server capacity. The Origin social servers sometimes have problems keeping things running smoothly. And don’t worry you’ll probably find that 100 friends is more than enough, even during special events.