RPM refers to the number of crankshaft rotations per minute Each piston/rod combination is connected to the crankshaft, so each assembly goes up/down once on each rotation. So, for a single cylinder, that one assembly goes up/down 1,000 times at 1,000 RPM. On a V-8, each of the 8 rod/piston assemblies goes up/down 1,000 times at 1,000 RPM.
As was said, in a 4-stroke engine, that means 500 sparkplug firings for EACH cylinder at 1,000 RPM.
The number of up/down cycles per minute changes with RPM - but the net effect of more cylinders is that there are more power pulses, more firing events, during each rotation. So, on a single cylinder, there is a power pulse every 720 degrees of crankshaft rotation (for a 4-stroke). That's why they vibrate so much - big pulse every other rotation. On a 4 cylinder, then, you get a power pulse every 180 degrees (on average, crank design matters) - much smoother. Now, a V-8 gets a power pulse every 90 degrees (again, on average, the crank designs vary) so it is even smoother. And the V-12? Every 60 degrees of crank rotation sees a power pulse, so it's very, very smooth.
On that 4-stroke V-12 (the extreme case vs. the single cylinder), you're getting 500 firing events PER CYLINDER per minute at 1,000 RPM, but because there are 12 cylinders, you're getting 6,000 spark plug firings/power pulses per minute at 1,000.
Take a look at YouTube - when I was a kid, I built a model called the "Visible V-8" - and I'll bet that animations of what's going on inside an engine like a V-8 are on YouTube these days...