Exercise execution is one of the most important components for achieving desirable outcomes with training. Before concerning yourself with how many reps you should perform you first need to master your execution.
There are a large number of categories that fit into this monstrous topic of exercise execution. When deciding to do an exercise consider these 3 features:
Motion and Position:
what is the movement you are performing? ie what joints are you going to move.
what position are you going to maintain? what do you need to actively hold still?
Contractile Control (Range of Controllable Motion):
within this movement what can you actually control with muscular contraction? are there big sudden changes in speed, or sudden changes in path of motion indicating you have lost control?
move through a range that you can maintain consistent control in.
Solid Stops and Starts:
on every repetition there needs to be completely concrete ends. Meaning when you begin the rep there is no acceleration and when you approximate the end of the motion there is quality deceleration, no bouncing or bobbing but a complete stop and hold.
Working on these three components of exercise execution will dramatically improve your outcomes.