My roadrace number is 89, that was the number the promoters had given me the first time I qualified for a World Superbike race, so I figured it is my lucky number. I was number 9 in '96 after finishing 9th in the Australian Superbike Championship in '95.
My modern motocross number is v61, it was the lowest number available when I called-up to register it (89 was not available).
I use 61 as my VMX number because it is now on the back of all my mx jersey's.
When I was racing mx back in the day, A & B grade riders in Victoria were assigned their number by their ranking, depending on how they scored points during the previous 6 months racing, my last couple of years racing before switching to roadracing I was 54. Victoria in the mid '80's only ran with 40 A graders and 40 B graders.