I get the spiral plastic colored leg bands from Cutler Supply. You might call them and talk to them about their metal bands. I'm not sure if they come numbered or if you need a punch for that.
http://www.cutlersupply.com/cart/index.php?main_page=index&cPath=6
I don't know how many you are talking about, but as long as the numbers are not huge, you can use a color coding system pretty well to cover a lot of different divisions. You have two different legs to work with and can put a few bands on each leg.
Say you go with two colors, yellow and blue. You can leave the left leg bare and on the right put:
bare
one yellow
one blue
one yellow and one blue
two yellows
two blues
one yellow, one blue, and one yellow
one blue , one yellow, and one blue.
two yellows side by side and one blue
two blues side by side and one yellow
Then do all this with the left leg one yellow. Then do all this with the left leg one blue. And so on.
Add a third color and you can see how the possible combinations grow tremendously.
I'm not going to argue against zip ties either. They work great. I just happen to use the spiral bands.