This is a common question; How many reps should you be aiming for with your exercises?
Well, as with most things... it depends!
Firstly on what your training goal is, and the goal of each exercise in your workout.
To train for a focus on pure strength, a particular number would be picked within the range of 4 to 7 reps. Here you would be lifting very heavy weights.
To train for hypertrophy focus (muscle growth), a given rep range would be used somewhere between 8 and 12 reps. Here you would use moderately weights.
And then for muscular endurance focus would be reps from 13 to 20 and beyond. Here the weights would be a lighter.
In my opinion, it also depends on what your training age is; that is, how long you have been training for - not just exercising, but actually training with a purpose, with a structured plan, for a specific goal.
Now as your not a strength athlete, your not going to be going in and doing say 5 sets of 5 reps on a few lifts for your workouts.
And for body composition purposes, doing 15+ reps on every set of every exercises with light weights isn't going to induce a lot of damage to your muscles in order for them to grow.
In my opinion, for a beginner to lifting weights, working in a rep range of 10-12 reps would be ideal. As this is in the middle, it allows you to learn the technique of the exercise for a moderate amount of reps, with a moderate weight to stimulate muscle tissue to grow. Rather than going too heavy for lower reps and not having enough time to learn the technique and get used to it, and also not going too high on the reps where form can break down easily from fatigue.
And then when you've got a bit of experience under your belt lifting weights, learned correct techniques and got a bit stronger, multiple rep ranges can be used for specific exercises in a given workout;
For example, you may choose a big compound movement, such as squats or deadlifts as your main lift for that session, where you can go heavier and focus more on progress overload (doing more over time, such as increasing weights), so may go with a rep range of 6-8, where can focus on strength but also build some muscle due to the amount of tension.
And then doing some hypertrophy based exercises with ranges of 8-10 or 10-12 etc, to work and damage (in a good way) specific muscles in order for them to repair and grow.
You could also throw in some metabolic work to stress the muscles with rep ranges of 12-15, or 15-20, 20-25 etc on exercises that work well for it, which is where you'll really feel that burn as lactic acid will be high, and the muscles will get that 'pump' from blood flooding into it - which is something done a lot in bodybuilding.
So, the simplest way to get started, is to decide on your goal and choose one rep range for your working sets, such as 3 x 8-12, so you know how many to aim for at least and at most, and select an appropriate weight for that, and then be able to know when to increase that weight if your overachieving that rep range.
If you have any questions on this topic, or are unsure, just drop me a message here and I'll gladly help you out