My understanding is that everyone is assigned a number although in some cases you may have more than one.
For example if you have a unit position and a council position (such as commissioner) you may have multiple numbers.
If you don't know what your number is, best way is to contact your local council.
I for one though do not know my number as I have not received a card in a few years and cannot find any old ones.
I did receive FOS requests sent to me that had a number but when I tried using that on a BSA site, they could not find a record of that number???