As long as you own the domain name, you can create as many subs as you wish. Your host may change for them though. I have a dedicated server, so I can have as many as I please :)
-Andrea
Your hosting company would have to make an addition to the server c-name records. Some hosts will not do this at all, some will do it for an extra monthly or set-up fee, and some hosts include it as part of their service package.
I've seen a similar one for finding RFCs, but I can't remember where.