Three (four if you want to count Lorri Johnson).
Are you advocating for any of the other three (Johnson passed away, so it probably wouldn't be her) to be the head coach simply because their number has been retired? Why on earth would the fact that his number is retired have anything at all to do with his qualifications to be the head coach?
Duke just hired a new basketball coach. They have several former players who have had their numbers retired who are currently coaching, including at least a couple with P6 head coaching experience. And yet they didn't pick any of those guys to be their next head coach. Odd that they didn't think that picking someone who had their number retired there was all that important.
North Carolina just hired a new basketball coach. They have retired the numbers of 8 players (and they have some pretty strict requirements to retire a number). They also have another 45 former players with what they call "honored jersey numbers". And yet when they hired a new coach they didn't pick ANY of those 53 honored former players. Odd that they didn't think that something like that was important either.
Retired numbers ARE, in fact, meaningless as to whether or not someone is qualified to be the head basketball coach of a P6 school.