@Cpayne32 Back in the day, F1 didn't have that personal numbers thing. In those days, the team of the defending world champion would have 1 and 2 (0 and 2 if the last year's world champion retired from F1, best example Damon Hill in the 90s), and then the numbers are given based on the Constructors Championship position from last year.
Since Ayrton Senna passed away in Imola 1994, the whole personal numbers thing didn't exist, and therefore there's no reason to restrict the number he ran back then. The only time where this is a thing nowadays is the 19 in F2, which Anthoine Hubert had in Spa before he died in that monster crash with Correa.