Thread:KusaNin/@comment-37657303-20181201191432/@comment-25156300-20181201195946

The way I do it is to first look at canon characters that would be around the same age. Ninja registration numbers will be similar for each student in a given generation.

So for example, Naruto's class has numbers like 012607, 012611, and 012618. You can see the last digits are close to each other, so the rest of the class would fill in around those numbers. Kakashi's generation had numbers like 009720, 010252, and 010829.

So then you can get an idea of the starting numbers (012 for Naruto's class) and pick ending numbers that haven't been used yet. Narutopedia should have all the canon numbers to help you. Also, different villages have slightly different ways to number. That's how I came up with my own characters' numbers, though there wasn't any canon Grass Village examples to go by. I used GR-# for them, though KU-# would have probably been more accurate, as it is Kusagakure. Then for the number portion, I just guessed how many numbers the Grass Village may have gotten through. (A smaller amount than Konoha, due to their size).
 * Suna has numbers like 53-004
 * Iwagakure has numbers like IW-09011