I think 2 is a perfect number of Capos. Being that they receive direct orders though indirectly from the Don having 2 is a check and balance from either one overtaking the Don. With 3 or more one could persuade others to maybe turn against the family.

Business bad? Fuck you, pay me. Oh, you had a fire? Fuck you, pay me. Place got hit by lightning huh? Fuck you, pay me.