I am at the stage where I can have a max of 15 villagers, how do I increase the maximum? Do I have to build another hut? My villagers have already built 2 huts and converted the large one into a school.

You should be good to goto a larger population. Once you get close the villagers will start building the next hut.

Oh ok, I thought I had to do something else to be able to have a larger population. Thanks!!

Shouldn't just keep at it.