First the 2lbs is an average. It comes from Fred Hatfield who says that a non-beginner, natural lifter cannot build more (baring some exceptions like being a genetic freak or regaining lost muscle) more than 25lbs in a year of training. And that is if EVERYTHING is done perfectly. So it averages out to 2lbs per month.
In reality it’s likely lower than that. Very rarely have I seen a non-beginner, natural lifter gain 25lbs of MUSCLE in a year. I would say that 10, maybe 15lbs is more realistic… but even then… let’s be honest and objective for a minute how many people do you know gained 40lbs of solid muscle (not a mix of muscle, water, glycogen and fat) in 4 years? I’m betting that few know people like that.
You can gain more WEIGHT. But it will be from fat, water, intramuscular triglycerides, added capillaries to support the muscle, etc. From experience with every pound of muscle added your body weight normally goes up by 1.5lbs even if you are not gaining subcutaneous fat. Gaining 10lbs of muscle without adding any fat would trust translate to a gain of 15lbs. And Most natural trainees to achieve that 10lbs gain of muscle would likely need to gain 20-25lbs.
Furthermore, like I mentioned the 2lbs (more like 1lb) is an average. It is possible to gain more tissue in a month, and on some month you will gain less.
As for how much can you make an individual muscle grow I can’t answer that. There are so many factors involved that even a supercomputer could not answer you. Sorry