College basketball jerseys can have only the digits "zero" through "five." This totals 36 numbers, assuming the numbers '0' and '1' are given out. Duke has never had a player with either 0 or 1. Then I suppose there is '00.'
FWIW on TV the UNC rafters look like a fabric store. There are a lot of retired numbers. Duke has retired 13 jerseys.
If needed, then I suppose that certain "retired" jerseys go back in circulation, probably in historical order.
sagegrouse