Is there a minimum number of people needed for an aircraft pushback?
Depends on the airline.  At minimum, you need one driving the tractor, and in unconfined areas with nothing to run into, this is sufficient.  In confined areas operators almost always require wing walkers to clear the tips, and someone at the tail if backing into a confined space or to make sure the pushback stays within any boundaries.

So the answer is, anywhere from 1 to 4, depending on the situation and the airline's internal policy.  Insurance requirements will have an influence on this in a lot of cases because the results of screw-ups are usually just insurance damage claims with nobody getting hurt.