You want to have enough instructors participating in the pilot to be able to tell whether what you're doing is a success, but you don't want to have so many instructors participating that you become overwhelmed, or if something goes wrong, that you can't fix it. Not every instructor who agrees to participate in the pilot will actually follow through, so it's good to have 3-4 signed on initially if you're running a volunteer-only program. If you have staff members working on your education program, you likely have the capacity to support about 10 instructors. Think about the other commitments on your time and determine what is the best number for your pilot.
One caution: Think more about how many students each instructor will bring. The 10 number is on the assumption that each class has about 20 students; if class sizes you're working with are closer to 100, reduce the number of instructors you work with.