Then there are others who can't wait to be asked, but volunteer before you are even ready. However you obtain your instructors, you should have a lot of them on tap. If you are constantly changing your offerings, you will have a better chance of members attending more than one session. You also want to have lots of instructors so that your instructors are not burdened with spending the entire session teaching. They should have some time between classes to prepare for their next class if they are teaching more than one, and also have some time to take some classes.
Many instructors enjoy the chance to learn something new as well as share their knowledge.