Mine of course is a longer story with a few kinks thrown in for good measure. Back in the Day as many people have mentioned, you used the last 3 digits of your AMA # as your number. My dad was at a race somewhere where this was being forced on everyone and he didn't have an AMA card or the money to get one for the race. So he "borrowed" Mike "Robert Deniro's look a like" Monomorono's AMA card for the race which ended in 163. My dad did pretty well in that race, and decided he liked the # and always ran it after that, never using his own AMA # after he got a AMA card of his own. Later on, after many injuries to my dad, my mom refused to let me "race" motorcycles and I just rode with my dad on practice days on various mini bikes while I was a kid. Once I was big enough for my my first full size bike, he got me a 1985 KTM 125 at age 15, which looked just like his 1985 KTM 250. He put 163 on it just like his bike, so he could leave it on the trailer and my mom couldn't tell the difference that it was my bike that I started racing vs his second bike as a spare……. and the number stuck with me then too. So I've been 163 for 30 years in a row now based on my dad pulling a fast one on my mom so I could start racing at age 15. Luckily, we've never had to race each other, because when we were about the same speed, he was busted up with a compound fractured leg for 2 summers while I was getting faster and better. By the time he returned to riding / racing after that 2 years off, I was the faster of the 2 of us and never had a battle of death with my dad on the track. Thank goodness, because I'm not sure if both of us would still be alive if we had tried racing each other at the same speed, I'm sure he would have taken me out and left me on the ground at that time.
When I go to Loretta Lynn's and need a 2 digit #, I always ask for #63 so I can drop the 1 and still be as close to 163 as I can. It's become my number that everyone just knows it's me and I don't think I'd run another number ever, kind of a tribute to my dad for getting me into riding and racing and even sticking his neck out against Mom, so I could race.