If you wanna build as much muscle as possible and are not concerned with putting on a little fat then eat a high calorie meal every 3-4 hours. Make sure its high in protein without having too much concern about keeping other macros in check. As long as your lifting is heavy with a good amount of volume and frequency you will still build mostly muscle. A few medium intensity medium duration sessions a week on the recumbent bike isn't a bad idea either. The best gains I ever made in my life was 19-21 (keep that in mind, I was young) and I gained about 30 pounds of mostly muscle. I lifted heavy 5 days a week on a 4 day split. So it was a different workout m, t, w, th and then Friday id do Mondays workout again. The following week Id begin with Tuesdays workout and continue the cycle. So if each letter represents a workout it looked like this:
A B C D A
B C D A B
C D A B C
D A B C D
An average day of eating was:
-3 eggs on a roll w sausage, cheese, salt, pepper, ketchup
-Chicken cutlet hero with melted cheese lettuce, tomato, mayo.
-couple of cheese burgers
workout
-double serving of chocolate milk (Skim plus has 11g protein per serving instead of 8)
-1 - 2 cans of tuna or canned chicken. As much as I could fit on a sandwich, the rest I ate alone.
A few disclaimers. As I said, I was young and my age, lifestyle, and training could easily use this many calories to build muscle. This is a basic bulking bodybuilding routine. Its actually funny typing this bc Im 34 now and been eating an entirely plant based diet the last 3 years. Still 225 pounds at 6 ft 2 but man do things change. Hope this was helpful.