The number of reference points required by Photo4D is determined by how many imaging parameters to compute. As a minimum, Photo4D requires at least two points with x,y,z coordinates known (Ref XYZ points), plus another known coordinate value, i.e., a point with at least one of its x,y,z coordinates known.
It is a good practice to acquire as many reference points as possible. The reference points should also be well spread over the measuring volume.
In many situation, one or more coordinate values of a point can be inferred from the other points. For example, if you know p1 and p2 are at the same height and the coordinate of p1 is (10,20,30), then p2 can be set as a "Ref Y" with y = 20.
Please refer to Photo4D User's Guide for more information.