Depends on the droid. C-3PO is a 3PO class droid. R2-D2 is an R2 class droid. So there’s only 36 designations for 3POs and 1,268 for R2s.
BUT, designations don’t always have to follow the standard scheme. For example HELIOS-3E is IG-86 class droid. And we’ve got Chopper (C1-10P) a C1 class. We’ve also got R0-GR, a B1 series; though my head canon is that all B1s are jammed Roger, so when they say “Roger Roger” it’s actually “Roger, Roger”.