So basically to get bigger you should try to keep your reps to a maximum of 12?

I usually keep the number of sets to 12 no matter how many exercises I am doing and the reps from 8-10 and 6-8. If I do 4 different exercises then I’ll do 3 sets each, if I do 3 different exercises I’ll do 4 sets each etc…

I recently joined the gym again and the personal trainer told me that I should be doing from 12-15 reps for maximum grow, but I really do not think this is truth... Can anyone show me the light here?