I have a trio and a quad of boars but certainly dont advise trying it, i have gone through many hours of bonding and some piggies were temporarily left without friends due to clashing with there previous mate because i added more pigs and it upset the balance.
I'm about to add a 4th pig to my trio but the risks are great so I'm still considering not doing it, the only reason i need to do so is that Ralph has becoe to dominant for his own good and needs a slightly tougher but reserved pig to take him down a notch or two.
The other issue is that you could potentially end up with all pigs single from fighting.
The cage sizes have to be large too, my trio are currently in a 3 x 4 C&C cage and my quad are in a 4 x 5 C&C cage, with boars and piggies in general especially big groups a second story shouldnt be counted and they should have the recommended amount of space on the ground floor alone.
Piggy politics are very complex once you start big boy groups, i would stick to pairs its much easier xxxxx