Let's talk about reps, as in repetitions, for a minute. The number of reps is how many times you're going to repeat an exercise before that set is done. I have a lot of clients who get caught up in the number of reps they have to do. Lots of times they won't be able to start the exercise without knowing how many reps they have to do.
I get it, the number of reps sets an expectation for the exercise. If you only have to do 4 reps of an exercise, it's probably going to be "harder" than if you have to do 15 reps of the same exercise. This is good to know.
Reps become a problem when you get fixated on the exact number. I often see people rush through a set just to hurriedly get to the prescribed number of reps. Or people will stop a set early (before they reach appropriate fatigue) just because they reached the number of reps.
If the world were perfect, you would look at the rep scheme as more a suggestion. As a matter of fact when clients ask 'how many' I'll often give a range. The range will be based off of whatever the goal of the exercise or session/program is for that day. The most common rep range suggestions would be as follows:
Power- 2-4 reps
Max Strength- 3-6 reps
General Strength- 5-8 reps
Hypertrophy (muscle growth)- 6-10 reps
Endurance- >10 reps
So depending on what you're trying to accomplish you'd pick the appropriate rep scheme.
The thing is you don't have to be stubbornly committed to whatever rep range you choose. Particularly if you're working out for general fitness, just trying to lose weight and get stronger.
So if not a particular number of reps, what should you be looking for/working toward?
On one end of the spectrum you need to be aware of technical breakdown. Technical breakdown is the point when the form of the exercise begins to degrade. This usually happens when you're fatigued. So if you feel the form start to go on rep 4 out 8 on your squats, stop. Rest and recover. Live to squat another day (or another set, you know what I mean). Technical breakdown is where injuries occur so avoid it as much as you can.
On the other end of the spectrum you need to do enough reps to elicit a training effect. So if you get through all 8 squats and feel like could've kept going you're probably not working hard enough. You either need to increase the weight/resistance or increase the number of reps. Again your choice will depend on whatever your goals are.
Your workout program should have appropriate rep schemes. The point is you don't have to be stuck on those numbers though. Use the numbers to help you make the necessary adjustments; especially when you're approaching technical breakdown.