Guinea Pigs... SUPER SOCIABLE!! You can have as many piggys as space will allow. Keep to all girls (sows) or all boys (boars). Groups of boars or groups of sows will tell eachother off a little until they work out their pecking order, this hardly ever leads to any harm and is more of a shewing off and nip than a fight. If you have lots of sows together I do recommend getting a single neutered Boar as he will naturally become the leader and cause less disturbance amogst all the sows.
Most of the time Sows will just go straight together and have a little nip at each other and play the head raising game to get into the order of dominance. However occasionally there will be a sow who doesn't want anyone else in her space. In this case you will need to bond them gradually. See the bonding bunnies post for ideas!