Hi Bill,
as Daniel already mentioned, the number and distribution of GCPs depend on the area you are a mapping and the accuracy you want to achieve.
Some general recommendations can be found here:
- Number and distribution of ground control points (GCPs) in corridor mapping.
- Number and distribution of GCPs in Corridor Mapping (community post).
As a side note, we recommend adding more GCPs in the area of interest. e.g. around stockpiles in case of volume measurements, or in case that there are big elevation changes in the scene.
Best,