Not to beat a dead horse here, but if you just want to put a number on your sail and not race with an organization that requires a sail number, then do so. Tradition holds that that number be the hull number of the boat. Usually the boats logo sits at the top, the boats model or length under the logo then the hull number under the model number, but you can do whatever combination you desire and still be "right".
If you want to race with an organization that requires a number on the sail, then join that organization [expect a fee] and they will assign you a number and tell you how and where to display it.
Hope this helps