Your relation number is your (new) number within the KNSB, with which you can register for competitions. This number has replaced all other numbers and starts with a 10. The number can be found by logging in to My KNSB (https://mijn.knsb.nl/login) after which it can be found in the top right-hand corner of the screen. You can also enter the number after your registration at Settings → Personal details.
No membership number and still enjoy the benefits? Then quickly become a member of a KNSB skating association of your choice. You can easily add your membership number later at Settings → Personal details.