Your boat's individual sail number should be your hull number, and that is part of the HIN. OEM sails will usually have the class emblem and the sail number. Sometimes a prior owner was too cheap to pay for having this on his new main. It's good to have it.
If only for the emergency when you have to call for a tow or a rescue and telling them to look for "the white sailboat" may not always prove helpful.
Getting a national-assigned number is good if you race in more formal events or sometimes in other venues. Local race committee's usually will not require it, but DO need a way to identify you quickly in a crowd.
Often, when buying a well-used boat, the original blown-out main is at the bottom of a pile of stuff including in the sale... and on its faded panels will be this information, or what's left of the numbers and emblem.
If you post up the actual HIN, we can help you decipher the info from it. Usually. Perhaps.
Fair winds!