I often get asked "Just how many GCPs do you really need?
My answer is, it depends!
The number of Ground Control Points (GCPs) will depend upon the number acres that are being flown, the shape of the site and what drone/camera/sensor is being used.
The very minimum is 3 that most processing software needs to start rectifying the images.
My minimum is 5, one in each corner and one in the middle for a 1 to 2 acre rectangular lot.
For a flat 5 acre site at least 8 are needed as shown below.
If there is a lot of terrain relief, then more GCPs are needed to accurately capture the grade breaks for computational purposes by the processing software.