In "This Is Spinal Tap" David St. Hubbins tells Marty DiBergi
...you saw exactly how many people who's been in this band over the years, 37 people's been in this band over the years.
Further information from a Spinal Tap e-zine:
While David tells Marty DiBergi during the 1982 filming of “This is Spinal Tap” that there had been 37 people in the band, only 20 have been documented. The lost 17 likely were short-term hires during the turbulent years of 1965 and 1966. In a 1992 interview with the Orlando Sentinel, Nigel mentioned that 12 drummers had preceded Ric Shrimpton, meaning that seven of the missing 17 were drummers. The current total stands at 42.