Can only one L50 exclusive skill be activated per mastery or just one at all on a character?
Only 1 on a character.
If you want to put points into two exclusives and switch between them you can, but only one can be active at a time. It would be a bit crazy to do so since you’re likely to need the points elsewhere, but there’s nothing game-wise to stop you having two or three: Soldier’s Oleron’s Rage, Menhir’s Bulwark and Occultist’s Possession or even four: Soldier’s two plus Arcanist’s, Inquisitor’s, Shaman’s or Necromancer’s if you can manage to get points into them all.
Thanks for the response. Then I can save the points to improve my pets instead of wasting on an exclusive in the occultist tree. I have one exclusive in the necromancer tree already maxed out.
Going for pet cabalist eh?
Master of Death all the way then 
It wouldn’t be very exclusive if you could have more than one active at a time, now would it?
The thing was not so clear. It could have been one exclusive per mastery.