if you want NPC followers as Slaves Mercs, Dogs etc, it's CH/2 - 1 If CH=10 it will be 10/2-1 =4 8CH = 3 6 CH =2 4CH=1 but you can have one merc with 3 Charisma so my maths is bad

3/2 = 1,5 - 1 it's HALF but you can have merc
Ofcourse if you want more than one with lol charisma you can choose Magnetc personality perk wiitch will allow you to take 2 mercs and one Player ( probably bug )
As players 2 Charisma it's 1 follower 3 = 2 followers and so on (CH10 = 9 Followers)