Although there are no regulations by VEX, a VEX team usually requires 3-6 people. But in that team, splitting roles is the most important thing needed to start a successful season. There are other ways to divide roles and work in harmony, but this is an example that our teams use.
A team captain will help with team coordination by a mile. The job is usually to organize meeting dates and divide the work. A team captain could also be organizing the engineering notebook and planning who will write in it. A team captain is more important if some team members are new to VEX and are still figuring out robotics. Don't forget that the team captain also has to take up a physical role of contributing to the robot!
Most teams have dedicated programmers in the team. Although everyone in the group should have VEX Code installed in their computers and know basic things such as how to download code into the robot, not all will have to be able to code proficiently. Because VEX Code does not allow for multi-user editing, usually one main programmer is enough for a team. A back-up is also recommended although does not have to be as proficient as the main, for when the main programmer is unable to attend events and such.
Everyone in the team will have to contribute to building at some point. The programmer can not program unless there is a working robot. But a head builder will be the one in charge of making sure the robot is being built as planned, and knowing everything from mechanisms to structures.
This role will be mainly for experienced teams. During tournaments and competitions, one or two members could act as a scout and grade other robots. This will come in handy when you rank in the top 12 and in the position to be able to choose an alliance. Although not necessary, it could also help the team learn from other teams as many are willing to share a lot about their individual robots. When scouts from other teams come to yours, be nice and answer their questions, they aren't spies!