The more players with a brutal personality you have in your formation, the more often your team will attempt to hit your opponent with brutality.
Playing brutal comes with a cost as your brutal player lose some focus on defending. So do not play too many brutal players at once, unless you are not afraid of your defense will stop all attacks.
The optimal bonus between making hits and also avoiding too much defensive penalty is to have 2-3 brutal players in your starting lineup , BUT depending on your opponent and their tactics the correct amount can vary between 0-5 for optimal usage.
Using more than 5 brutal players in your lineup is rarely smart. It will hurt your defense a lot.