Students should not need to know how to build a robot. This learning and discovery is one of the joys of participating in FIRST. Ensuring you have students dedicated to sticking with the program is essential.
One way to explain the commitment students need to have is to compare this program to a sports team or other school programs, like the school band. More than likely, a FIRST robotics program is new to your school. By using similar programs as a comparison might help everyone understand the time commitment better.
Up front, it is important for the team leadership to communicate to students of what the expectations are for student (and adult) members of the team. By doing this early and clearly, you will minimize problems later.
Gauge the appropriate number of students by the number of professional mentors you have. It is difficult if you have more than a five to one ratio of students to mentors. List all of the activities you need and want to do as a rookie team and this will help you determine how many students would be appropriate. There is no set number, but somewhere between 10-20 students your first year might be a target.
TIP The number of students is not nearly as important of the commitment of the students you have. If you have five students who will weather the storm no matter what, your team might be in better shape than a team of 20 students who show up when they feel like it.