Guinea pigs are extremely social animals, if you can not give the guinea tons of attention every day, I suggest 2 so they can keep company and happy. I started out with 1 and now have 25 of my own plus the adoptables.
You also have to realize the space they need for housing. They need room to run and kick kup their heels (yes they actually do that provided they have enough room). Their cage has to be a solid bottom, no wire bottom cages. Wire bottom can break toes and legs and cause a horrid condition called bumblefoot.
They need/should be weighed weekly to watch for weight loss. If there is more than 4 ounces of weight loss in a week get them to the vet now because they are at death's door. Illnesses vary with the pig, some common things are; abcesses, cysts, URI, UTI, malocclision <sp>, and various other things. They are not an easy pet to care for and once any of these things pop up you need to get them to a knowledgalbe vet to get them taken care of.
A guinea pig can live 8-10 years provided they are given proper care.