Simple answer:
Decide on which number you want. Ring the secretary of the club(s) you want to race with, and ask if that number is taken.
If you're the first, then you can 'bags' the number, get your numbers made up and live happily ever after.
If someone else has already taken the number, then choose another and start again.
I went through the same stress a few years back, but its really a non-issue.
Numbers that are 'always' taken:
88
69
and anything that can be easily made from electrical tape.