Meaning of numPixels parameter for unsupervised clustering in Google Earth Engine

I did an unsupervised classification with google earth engine. numPixels parameter change the classification result hugely. I am very curious about how this parameter work and what does it mean and what is the optimal values for best results. My clustering code below here.

var training=clipdata3.sample({
  region:geometry,
  numPixels:101, 
  scale:10
}); 
var grupo= ee.Clusterer.wekaKMeans(3).train(training); //K-Means Method with 3 classes
var classified= clipdata3.cluster(grupo);