The IHCA recommends that you pick the middle four numbers in your hull number, which is engraved on the front crossmember and on the transom. They also suggest just picking a number and using that until you get to a regatta and there's a duplicate it which case you can add a "1" with electrical tape. Good thing you bought lots.
Both bodies talk about sail numbers but in practice it is expected that you will put numbers on the front of your hulls. Your choice. IHCA specifies that sail numbers will be 10" high. Since they don't talk about hull numbers I don't know. You can get numbers from a Hobie dealer or use the digital eight numbers sold for Lasers. I think sail numbers should be set at the level of the race committee, about half way up the sail. I'd put an image of my sail if I could figure out how.
You need two sets of sail, or hull, numbers. Outside right and outside left on the front of the hulls. If you're going to put them on the sails they should be offset from one another with the starboard (right) side higher. The sails are translucent enough that the numbers become almost unreadable if they aren't offset.