How many you can keep depends on how much grass you have. With just a couple it might be more difficult to send them off when the time comes as they tend to become pets, but there's no reason why two wouldn't work - as long as you don't have just one.
Also buying in lambs means either orphan lambs and you don't know why they were orphaned or what feeding/care they have had between being orphaned and you getting them, so vets bills could be an issue. Or if you buy lambs at weaning they won't be doing much grass eating before it's time to send them off for slaughter.
If you have the ground, why not keep a couple of ewes of your favourite breed and get a crop of lambs off them every year?