I sometimes have a lone male, but I find that he's happiest when he's baby sitting the grow out boys - so I try to keep someone with him. I really think that they benefit from the company of another rat.
I strongly believe rats need a buddy. Whether they are growing out new keepers, or retiring, I would advise having a buddy system for all of your rats. They do get depressed and that can result in problems with health. They are very social creatures, and if you can't afford to have 2 together at the same time because of space reasons, at least interact with the rat everyday.
If they are being raised up, it may be best to give them a buddy regardless. Nothing like being a weanling alone all day