You can put any number you want on your sail, as long as there is not an identical number racing against you, you'll be OK. There really is no "Official" numbering system anymore like there used to be many years ago when it was just Hobie's and Prindle's and they used hull numbers like the Mono guys still do.

If you are going to join a one-design type class (I don't think the Prindle 19 has one at this time) you would have to check with the class registration people and see if anyone else already has the number you want to use, but other than that, pick your favorite "Lucky Numbers" and use those. Oh, and it could be just a single number, or two, three, what ever. We have had two boats show up with the same sail number at a few regattas but a quick modification with duct tape will fix that. 1 becomes 7, that type of thing.

Good Luck!