There seems to be a general assumption that everyone should be doing 3 sets of 8-12 repetitions for your weight training workout, but there are a couple of key points that I want to bring your attention to.
 
First, SETS: there is a body of research that concludes that most of the benefits of resistance training can be achieved by doing one set to failure and that additional sets, while they do have additional value, have a diminishing return (i.e. doing 2 sets does not give you twice the benefit of doing 1 set. It gives you less.) Of course how many you do in your program depends on your own individual goals and current training status. I typically use between 1-3 sets with most of my clients.
 
Next, REPS: Low reps and high weight deliver greater strength benefits. High reps and lower weight give greater muscle endurance. Most general conditioning tends to fall between that, somewhere between 8-15 reps. There is no magic number here, no lightswitch that says, if you do 7 reps it’s strength and 8 reps it’s general conditioning. There’s almost always a combination going on. 

Now here’s the important stuff about reps:
• Any of these repetition ranges will “tone” the muscle. “Tone” is what active muscles feel/look like.
• None of these are magic “get leaner” ranges. Getting leaner depends on your total program and your nutrition.
Whatever you or your Personal Trainer chose for your top rep, that is NOT where you should stay. It’s a graduation number. i.e. if you’re using 10 reps as your number (You could easily choose 13 or 14 as a target number), when you can do 10 reps in perfect form, you graduate. Raise the weight by the smallest increment that you can. Next time, if you can do 10, raise the weight again. Keep raising by small amounts until you reach a point where you can’t get 10 reps. That’s your workout weight. Stay with that until you can get 10 reps and then, guess what, you graduate. Raise the weight. Keep the progress happening.

To continue making progress toward your health/fitness goals, you must continue to up the challenge. Once the challenge is no longer there, you can even lose a bit of the benefits that you had already gained.

Good luck and keep pushing yourself. 

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