10*12= 120 sq ft
If the chickies will have access to a run all year round (i.e. won't have to live in the coop for days at a time when winter weather precludes them going outside), then 4 sq ft per chicken is ok. Otherwise, if there will be times when they are "cooped up," so to speak, then 10 sq ft per bird or more is the recommended space.
Your current stocking rate is
120 sq ft coop/ 16 current chickens = 7.5 sq ft per chicken
If you up that to 30 chickens, then the rate is
120 sq ft coop/ 30 chickens = 4.0 sq ft per chicken
Bigger is better, especially if they hang out inside a lot. The smaller the coop, the more frequent cleaning it will need.
My coop is 16 x 16 = 256 sqft / 27 chickens = 9.48 sq ft per chicken - a little on the cramped side on days that they have to stay in. In our area, that's usually only a few days here and there through the winter.
Hope this helps.
PS If you find 16 hens fun, you will find twice that many twice the fun! Good luck and many chicky blessings to you!