harlow thar... How come everything also 20lbs so ngam wan? When we say do 12 reps, it means go for the heaviest weight you can do with proper form for ~12 reps, ie. your 12-rep max weight.

Assume this situation, you are planning to go for one set of 12-rep. You pick up the 20 lbs dumbbell and begin your workout. At the 12th rep, you still feel comfortable enough to go on for another 4-5 reps. In this situation, it means that the weight you are using (20lbs) is too light and you will have to increase it (assuming that you are aiming for 12-rep exercises for that particular set).

So basically if you can go heavier, don't hold yourself back. If you can't, don't force yourself.

Nono, don't get me wrong, I am not recommending you to do 12 reps. I am just sharing with you on how to use the correct weight for whatever rep range you decide to do

But just FYI, from what darklight (he's an experienced bodybuilder on these forums) says, 8-12 reps is the optimum number of reps for muscle size growth.