Turn left and learn the rules.
A font, with regard to the number on the back of uniforms, is said to be readable if ...
it is one of a set list of readable fonts
it can be read easily at a distance of 20 feet (6 meters)
if you stand two players on the same team next to each other, you can see there is a difference
it can be easily read and distinguished from the other players' numbers by the officials, other players, and fans.