Good antiox/ ineffective lean muscle builder
Quite seriously, NAC is one of the most important antioxidants for athletes—the benefits are numerous and the research rich. Its effect on muscle trauma caused by lifting weights or intense cardio training is exciting. A close second (and an occasional first) in importance would be its role in detoxifying the body and helping keep the liver in "like-new" condition. Its anti-aging benefits and ability to aid in joint health are great value add-ons.
But to answer your main question, it does nothing for increasing muscle mass, besides possibly reducing muscle wasting.
Optimal levels are dependent on many variables, including amount of exercise and other specific situations when its ability to protect liver health may be important. Some reports have indicated the upper limit for effective NAC supplementation is 2,000 mg daily.