Lovebirds are AMAZING. I'm rather fond of them and hope to have one again one day :)
Like all species of parrots, lovebirds can be singly kept. You just want to be sure to handle them frequently and allow them plenty of playtime, just like you would with any pet. It's really up to you whether you should get a pair or not. Pairs don't always remain loyal to each other, so just be prepared to house them seperately if that happens.
I would caution against housing a lovebird with another species of parrot -- they can be pretty territorial, even with their own kind.
Vera Appleyard, who wrote The Lovebird Handbook, has an informative site with articles about pet lovebirds:
Parrots Senegals Amazons Lovebirds at Parrot Parrot
The African Lovebird Society also has some pet-specific articles that might be of interest:
Pet Spotlight!