People usually do sets of ten or twenty reps on a certain machine, but this number is unlikely to be what the body really needs. The perfect point to stop could be after eight reps or seventeen, on any particular day, depending on your energy levels.

You have to be well focused to catch that point. Your muscles will tell you when enough is enough if you listen to them. This can keep that enjoyable movement from degenerating into strain and tension. Letting your muscles decide when to stop a set is much more sympathetic than mechanically pumping out twenty reps just because that is what you always do.

As I said, I go to the gym to let my body play, as if I were taking a dog for a run. A dog will slow from running to walking, or will sit down and stop, at exactly the right time his body wants to. He likes to run but he also knows when to rest. He doesn't tell himself, as humans do, "I'll stop running when I reach that tree in the distance." If you let your body decide when to stop and start, and how much to do, you'll feel relaxed and energized when you walk out of the gym, rather than exhausted.

Breathe easily and do the reps harmoniously.
Be as relaxed as possible while working the body fully.
Notice when you start straining and losing that relaxed feeling.
Wait for your body to tell you the exact time to stop.

Eric Harrison in Flip the Switch