Guinea pigs are sociable animals, and unless you want lots of baby guinea pigs (which has to be considered very, very carefully) you should keep guinea pigs of the same sex together.

Some males get on well together, but some will not! The best pairing here is two males, who have been raised together, with no female contact. Two females generally get on well.

But there is no hard and fast rule about this, and some guinea pigs do actually prefer to be kept singly, although it is usually not a good idea to keep one alone by themselves.