You can have 8 time the same char if you change his/her name. It will cost you a lot of money but it's doable.
As stated above, you can have 8 time the same class it isn't an issue, the real limit is :
- You can't use as merc the char you're curently playing (I mean use him in the group).
- 8 is the max slot you can't have more.
Also as stated above, the merc (if it's a campagne one) will wear the armor equiped on him when you'll set him as merc, you won't be able to change it on the fly.
I won't talk about merc done with pvp char since mine are made on my pve chars...