Two males, to keep respectively other company, two females tend to fight. Guinea pigs struggle to live on their own, they get bored and lonely, sadly you'll find whether one guinea pig dies, the other one dies shortly after. I had 2 guinea pigs sadly they didn't last as long as they should, solitary 2 and 3 years, the problem with guinea pigs one minute they are fine, then the next they enjoy a heart attack and dead. Sorry to be so negative, but the pet store will probably tell you alike.
Answers: 2 guinea pigs, any 2 males, or 2 femals from the same litter. if you get a mannish and femal they will breed,you don't you probably wouldn't like that, Guinea pigs tend to be lonely, so get 2. :)
Definitely two - piggies love the company of their own species. They MUST be of one and the same sex!