So, the formula for the amount of merc/slave followers is Charisma -1/2.
This means that you can carry 4 slavers at 9 charisma. However there is a perk called Magnetic Personality which gives you an extra slot for a merc/slave/player.
This means that with 9 charisma and the Magnetic Personality perk you can carry a stunning total of 5 mercs/slaves. However here comes the problem.
Somoene with 10 charisma cannot take the Magnetic Personality, this means that he will be stuck at 4 slaves/mercs. This means that it is actually more benefitial to take 9 charisma instead of 10, while 10 IS supposed to be better. This is something that should be fixed.
I have 2 possible suggestions for this:
1. Give people with 10 charisma the ability to take the Magnetic Personality perk.
2. Increase the amount of merc/slave followers for people with 10 charisma to 5.