I don't know how many exist for each make, but with the brands we have and brands I've worked with before there was anything from 6 to a box of 20 and you would up date them every year. Some of the brands have more that 70.
Some brands don't offer lock nut boxes and as such are unique because were not going to keep buying boxes of lock nut sets to hold onto a key for them. So while they do repeat the chances of some one getting the right one on the street and robbing your alloys are slim. It's like keys numbers.
When I worked with Ford, a man who owned a '99 transit and a '05 Ford mondeo had keys with the same number and he was able to open the doors on both (wife found out by accident when she opened the mondeo door but couldn't start the car as she had the transit key).
Also when I worked for Toyota, a man came in complaining that his neighbour was able to open his car door (when he locked his key inside) with his key. Turned out again somehow his neighbours '88 corolla was had the same key number as his '96 carina.
These codes are unique as far as they reckon you'll never meet a person who's got the same lock nut as you but there not unique.