Actually, on some servers it works, and on some it doesn't, strangely enough =/ FLAB had a server not work on a map with 14 characters (not including .PMS) that started working once it was shortened to 12, but I've had a server work with 16 (not including .PMS)
For now, 12 to be safe, 16 if you want to risk your map not working in some cases.