Most likely, it will depend on the location where you will teach your class.
I teach at three gyms, and all three gyms pay a certain fee per class. I get paid the same amount no matter how many students attend each class. One gym pays every two weeks and the other two gyms pay once a month. Each gyms pays a different fee per class, but I don't mind that one gyms doesn't pay very well..because I love teaching classes.
I also teach at two community centers. One community center pays like the gyms...a certain fee per class. I get paid there once a month.
The other community center is on a commission basis. I collect the fees from the participants (who can pay by the month or by the class), and at the end of the month, I total it all up and pay the community center 20% of the money that was turned in for that month. One advantage of this type of class is the more students in your class, the more money you will make. It can also work in reverse, small classes do not bring in much money.
Find your location, and then talk with the person in charge to find out their means of payment. As you can see, there are several ways to get paid. One instructor whom I know..gets paid a flat fee per class, but she also gets paid more according to how many are in her class, such as a couple of dollars more for every additional person over 10. Classes that are paid in this manner and on the commission basis can really encourage us as instructors to promote our classes.
Hope this helps, and I am sure there will be others with more options.