You mean you want a frame counter. FPS is frames per second.
Add this expression to the source text property of a text layer. It comes directly from the expression menu in the timeline:
timeToFrames(t = time + thisComp.displayStartTime, fps = 1.0 / thisComp.frameDuration, isDuration = false)
If you need leading zeros let me know. It's complicated but it is doable with expressions
I would suggest that you pick a monospaced font and set paragraph to right justified
You can also use the effect "Timecode" and set its output to frame numbers.
Hi Rick, thank you, this helped me a lot!