As a fictional band, they have a fictional list of former members, all part of their parody format.
The reference to lower number will be the actors or celebrities who have actually played with the band. The others (all deceased) are made up, with deaths both comical and often parodying real life artist deaths.
To answer the 2nd part of your question, the number of actual people who have played drums with Spinal Tap appears to be 5 - Gregg Bissonette, Gary Wallis, Jody Linscott, Mick Fleetwood and Todd Sucherman.
Whilst Wikipedia list 9 known fictional drummers plus another 9 "others", all of whom are dead, I don't think there is an upper limit to fictional deceased drummers. David St. Hubbins has been quoted as saying:
Well, we've lost count. It got spooky. We decided we were going to
make an entire album with no live drumming at all--just a computer
program. And you know what happened? The computer crashed. And we
said, "Walk away. Walk away."