Question: is 12 reps too many?
Answer: If you're training for muscle size, choose a weight at which you reach muscle failure in the 8-12-rep range. ... That means if you can do only 6-7 reps, the weight is too heavy, so reduce it on subsequent sets. It also means that if you can do more than 12 reps, but simply stop at 12, that's not a "true" set.