Question

What is the minimum number of points required to make a plane?

Answer 1

Three points not lying on the same line define one and only one plane that they all belong to.
This is an axiom of geometry.

Explanation:

The fourth axiom of incidence by Gilbert - the most solid foundation of geometry - states:
For every three points A, B, C not situated on the same line there exists a plane α that contains all of them.

The fifth axiom of incidence states:
For every three points A, B, C which do not lie in the same line, there exists no more than one plane that contains them all.

See the Web site
https://en.wikipedia.org/wiki/Hilbert%27s_axioms