They say that 3 is better than 2, although I can't say as I've noticed much difference. They're a herd animal, if that helps any. (Do note that you can only have one boar in with girls, though. A single-sex herd of either sex is OK, though.) They'll eat proportionately more food and require more space, but it's not linear with the number of guinea pigs. (ie, doubling the number does not necessarily require twice the space.)