I think we need the denominator here to put '20' into context. 20 out of 100 million possible mutations is, meh, I'll take those odds. 20 out of 100, well, that's something else entirely, isn't it.
Edited because a correction by another user helps to clarify - it is not about the 20 mutations (which they created a mutant spike will all 20 of them!) though, it is more about what was and was not effective in defendening against the mutant spike.
Coronaviruses are RNA viruses, so there's only a single copy of the genetic material, vs DNA with two copies. The result of RNA viruses is that they typically mutate at a much higher rate than DNA viruses. There are viruses that mutate faster than Coronaviruses, though.