If you don't have a 1 rep max or personal best recorded then you can use a RPE system, rating of perceived exertion. The scale for this is 1-10. For example if you are using RPE 7 for 12 reps, this means you have roughly 3-4 reps left in the tank after the 12th rep. If you are using RPE 9, this means you are struggling by your last rep and can MAYBE push out 1 -2 more.