For maximum muscle gain you are suppose to lift 'heavy' weights but im confused as to how heavy it is suppose to be. I got told that you are suppose only lift as much as you can and do between 4 - 6 reps. Yet others say you're suppose to do weights between 10 - 12 reps
Some of the reason the advice will vary is because peoples goals vary. Powerlifters and bodybuilders generally lift differently (number of reps and amount of weight). My 2 cents...work your way up to a weight where in the 4th set you can BARELY get 8 reps. The big guys I know use 12/10/8/6 rule...heavy as you can and get 6 reps. If you use LAM's formula and your one rep max on benchpress is 100lbs then you will want to work your way up to the 80lb mark during your workout. But, i doubt you will be able to get your 80%of max weight repped out very many times. Try it out, see what works for you by sticking with a certain routine for a few weeks, if you dont like it switch it up. Good luck.
There is no one "right" answer. Different things work for different people. For me, either 8-10 reps or doing 12 reps and more volume work best. Low reps when I did them did nothing as far as muscle size went.
Im after gaining muscle mass and overall size, so 10 - 12 reps would be the better option?
As I said, there is no "right answer". It may work for you, or it might not work for somebody else. If you haven't tried it, do so for 4-6 weeks and see if you see a difference in your physique. Also realize that gaining mass and size is also a function of eating properly, so you have to get enough calories, protein and carbs to fuel the body so it can grow.