There should be a rule. Only numbers from 1-25, depends, i don't know how many players can there be. Id a club decides, to retire a number,it should be allowed, to have also a number 26, in this case...etc...
Idiotic numbers like 80,84,99 have no bussiness in football. If the players don't like it, they can more to the american soccer league, nad play under the number 99...idiots