How many reps should you do?
The amount of reps on an exercise should be viewed as an approxiamate range.
for example if an exercise says 10 reps, you're aiming for ABOUT 10 reps. If you can go past 10 with the weight you are using, keep going and don't stop at 10.
If the goal is 10 and you get 12, that's still perfect.
If the goal is 10 and you get 15 reps, you can probably go up in weight next set.
If the goal is 10 but you only get 6, you probably need to go a little lighter next set.