It takes a lot for a team to decide to retire a number.
NFL rosters are so big and there are only so many numbers, which is why it was surprising when the Buffalo Bills announced the team plans to retire Thurman Thomas‘ number in 2018. He will become the third Bills player to receive that honor along with Jim Kelly and Bruce Smith.
This article on Bills.com from 2011 explains that while only a few numbers are retired, there are still those that aren’t given out anyway.