Isn't it too restrictive if normal (without 60 speech) guy with 4 charisma can't even have single mercenary? What about a little protection of tents for lone players having problem with natural perception or taggers exploiting? With this system merc is something extraordinary for special builds. Why not to make slots system. Has been posted before.
Slot points (how many you can own in one time globally):
Charisma relation: CH-1
Becoming a Slaver: +2
Magnetic personality perk bonus: +2
Speech bonus: (Speech-30/30) (-30 because of starting stats, hope it's not possible to have 0 speech)
If CH>2 charismacheck=1, else charismacheck=0
General formula: (CH-1 + slaver*2 + magnetic*2 + speech-30/30)*charismacheck
Slave 1 slot
Dog 1 slot
Brahmin 1 slot
Mercenary 2 slots
Super mutant 3 slots
Following NPC formula: (CH-1)/2 (present) - how many you can have traveling with you.
Preview - global NPC owning:
10CH, magnetic personality, 300 speech, slaver -> 22 slots - unbelievable slaver build
10CH, magnetic personality, 180 speech, slaver -> 15 slots - power slaver build
6CH, magnetic personality, 120 speech, slaver -> 12 slots - typical new slaver build
7CH, magnetic personality, no speech points added/tagged, slaver -> 10 slots -> typical old slaver build
5CH, magnetic personality, no speech points added/tagged, slaver -> 8 slots
5CH, no perk, no speech points added/tagged, slaver -> 6 slots
3CH, no perk, no spech, slaver -> 4 slots -> minimum slaver build
3CH, no perk, no spech, non-slaver -> 2 slots -> 'high' charisma combat power build
Generally I would prefer Slavery as a profession unrelated with mecenaries where each level would make you can enslave better types of NPC - still without overpower.