Well, I'd rather you get a premium membership and read my book Geometric Metrology, Dimensional Tolerances Inspection and Practices in Manufacturing Fundamentals ---> https://www.engineersedge.com/geometric-metrology.htm
But, I'll give you a short answer.
The most correct answer is that all surface elements should be measured and verified to be in tolerance conformance to the specified geometric dimensions and tolerances. However, this is not practical in the real world. When using a CMM, the number of measurements points is a matter of confidence or repeatability of measurements actually taken. In general, parts can be categorized into three groups:
1) Low (unknown) confidence in quality and repeatability.
2) Medium confidence in manufacturing quality and repeatability.
3) High confidence in manufacturing quality and repeatability.
Ultimately, the inspector or quality engineer needs to decide where a particular part lies and make the minimum number of inspection steps to ensure tolerance conformance.
For category 1, low or unknown quality and repeatability, the inspector should consider increments of measurements around 10 x of the specified geometric tolerance range. For example, for a flatness tolerance of .005”, increments equal to .005 x 10 = .050 could be made. This may seem like a lot of measurement points, and it is, however until an understanding of the repeatability of manufacturing as it relates to quality is made one must ensure dimensional and tolerance conformance.
Category 2, measurement of not less than 50 x of the specified geometric tolerance range.
Category 3, measurement of approximately 200 x of the specified geometric tolerance range.
It is also recommended that quality organizations define a standard practice for all inspectors to follow in regards to number of required measurements points or data sets.