When I first was getting into Folk music in the late 70's I had a piece of advice on this subject from the great Martin Carthy, he said try to learn 10 songs very well, then add the odd new one when you can sing all the ten by heart. I'm 65 now and I have a collection/repertoire of over 600 and I've probably performed 400+ of those. but the 10 song advice was invaluable so I'd say a minimum of 10 and a limit of as many as you like ;)