The acronym RPM stands for Revolutions Per Minute. It's the number of times something turns in a minute. In cars, it's the number of times per minute that the crankshaft turns. It is not the number of times per minute that a cylinder fires.

The plural is already there, within the expression, on the end of the word it relates to.

To say "RPMs" is to say (excuse us while we struggle to pronounce this) "Revolutions Per Minute ssssss"

It doesn't make sense.

So don't say "RPMs" no matter how often you hear people claiming to be e.g. car people, say it on TV. It's wrong.