Okay, here's the scoop according to hubby (if he's memory doesn't fail) ...
In today's football, a player is assigned a squad number for the season (or possibly longer). Generally squad numbers reflect the position you play, but in more modern times, that's not always the case.
Back in Bell's day (if hubby remembers correctly), players were assigned squad numbers at each game depending upon the position they were playing
in that particular game. So if a player was a striker in one game and a midfielder in another, his numbers would not be the same.
This may be the reason why you are having such trouble finding the number. Have you tried Man City's site? Since he was such a hero there, I'd be surprised if they didn't at least have something showing one of his numbers.
Also, when I want references for players, I generally check
Soccer Base, but sadly they don't have (any of) his number(s). You might want to try searching Google Images in hopes that there's a picture of the back of Bell's shirt.
Sorry I couldn't be of better help.