Need help with statistical theories

Hello everyone, I am a student in environmental hydrogeology,
I am working on an area of 10km2 (5x2km) and in the study area there are about 18 water points. I had to Selection 6 sampling points out of the 18 points and then carried out a multivariate statistical study with the 6 points.
But unfortunately my potential reviewer disagrees with my data processing and states that a statistical study (PCA and correlation matrix) is only feasible from 20 samples or more.

I would like to have strong statistical arguments to defend my points seen, because all of my work depends largely on statistics ... please help me.