Your sail number should be marked on the boat. If you have a past '71 boat it has a HIN which contains the sail number as part. Older boats have hull number and class numbers on the builders plaque, and the latter is used on the sails.
If you absolutely don't know which number applies to your boat, join the DaySailer Association (DSA) at
https://daysailer.org (while they finance this forum, it's separate from their main websiet and needs a separate registration / login).
Contact the secretary and she can assign you a number to use.
Placement and size is given in the Bylaws - found on the same site.
Any competent sailmaker can sell you both logo and digits, but you may be able to find them online as well.