There are a lot of important things about football. One of the things at the very top is unquestionably what jersey number each player wears.
Numbers have to fit. They have to make sense. Most importantly, they have to be awesome. Whatever number a player wears has to check all of these boxes with regards to the position they play, the team they are suiting up for, and their style of play.
Speaking generally cornerbacks look best in numbers 21 and 24. There are certainly exceptions as the number 20 can also work and the old Terence Newman fan in me loves number 41.