"and unless you're really careful the numbers will fall apart every time you remove them.
"
Can't believe you re-use your numbers!! They only cost about 10p each fer christ sake.
If you're running in different classes and there is a number clash what New Era try to do is increment your number by 100, so for example you might be 42 in one class and 142 in the other - that way it is easy to change.
I guess you must all have your bikes built, track tested to set up gearing and base suspension settings and just waiting for the off - and that's why you've got time to worry about numbers!