Elephant Knight

I think this is the right place to post this... I'm not in the 501st, not sure if I want to actually join. Fairly certain my outfit, when complete,, probably wouldn't pass inspection (Mostly in the black undersuit). But when people do join, how do they get their number? is their a specific set of rules when choosing, or can you do whatever you feel like? Most numbers I have seen seem to follow a pattern. Or do you get assigned a number? EKnight