The thing is that growth comes in spurts.
You cannot extrapolate how much muscle one can build in one year. For a short period you might build 1lbs of muscle per week ... for a month, adding 4-5lbs lf muscle (maybe 6-7lbs of body weight) then your weight will not go up for another 4-6 weeks then you can again.
And it\s impossible to predict when these spurts will occur and how many you will have.
That's why some people will gain 5lbs in a month and already see themselves being 40lbs bigger at the end of the year... only to end up being 7lbs bigger after 12 months.
In my experience, in a non-beginner and non-performance enhancing drugs user, gaining 10-12lbs of muscle in a year is really good. I'm not saying 10-12lbs of body weight and not even lean body mass, but actual muscle tissue.
Total body weight is muscle + fat + water + organs + skeleton + glycogen (etc.)... for our purpose total body weight will be affected by muscle, fat and water gain/loss
Lean body mass is everything not fat in the body (so it also includes organs, skeleton, glycogen, water)... but fluctuations will come from muscle, water and glycogen gain/loss
Muscle mass is the amount of muscle tissue you have.
A normal male has 30-50% of his body weight in the form of muscle. So if you are 190lbs you likely have anywhere between 55 and 95lbs of muscle... in that context "only" 12lbs of muscle is pretty significant since it represents as much as a 22% gain.
The thing is that people have unrealistic goals because water and fat gain can give the illusion of more muscle growth. Someone will claim to have gained 15lbs of muscle in 2 months... in reality that person likely added 4lbs of muscle, 4lbs of water and glycogen and 7lbs of fat. The thing is that when you are of a normal level of body fat, you will not look fatter, so it is easy to assume that all your gains were muscle.
So when it is someone else that tells you that they gained 15lbs of muscle in two months, and you only see them in a shirt, it is easy to believe them and start to think that this is a realistic gain.
I've seen people gain 15lbs of muscle in two months... but at the end of the year they will likely only have gained a total of 15-17lbs which is A LOT.
For a normal, non-beginner individual, 10-12lbs of pure muscle in one year is VERY good. If that person didn't go crazy with the food that might come with a total weight gain of 15-20lbs due to some fat gain. Some will add 30lbs of scale weight, but that is only because they gained mnore fat.