If your tank is 22"x18"x72" And the Tangs were approximately 4" long when you got them,You could "conceivably" put 4,752 'Tangs" in your 120...
But they would be quite uncomfortable...if you cut that number in half they would still be uncomfortable.
Same thing if you cut that number in half again...and again...and again...until you got down to TWO Tangs in a 120...
Then they would be comfortable...However you could add "other' fish in the tank with them...
I have TWO Tangs in my 125 and they pretty much use up ALL the space not filled with Rock for swimming...And they are pretty happy.