Actually, two is good enough for happy lambs. They'll be each other's best buddies. And one lamb shouldn't cost much more than two, especially if most of their feed comes in the form of grazing.
As for fencing, I honestly couldn't help you there. I keep goats (their social/herd needs are the same), and if a fence can't hold water, it can't hold a goat, hah. It seems like a lot of sheep respect electric fences, though, if they're trained to it. I have seen many sheep contently fenced with hot wire and make no attempts to escape. Goats with hot wire doesn't work so well, they'll just take the shock to get at the nummies on the other side (even if you crank up the power, omg). The square field fencing should hold them just fine, too.
Yea sorry if I initially sounded harsh. But there are so many people out there dead set on keeping one goat, or one sheep. Or keeping said goat or sheep in their chicken coop. Or other things that are definitely not in the best interest of the animal. I won't sell my goats to anyone intending to keep a single animal. They must either buy a pair of goats, or they must already have other goats. A lot of sheep breeders are the same with their lambs, though you can find ones to sell singletons, even if it isn't healthy for the animal.