K means how to pick up K?

A general method

Elbow method (clustering)

Elbow method[4]

More advanced method

“It is often ambiguous and not very reliable, and hence other approaches for determining the number of clusters such as the silhouette method are preferable”

Silhouette (clustering)

Silhouette method

Reference

[1] How to Choose the Number of Clusters | Advanced Statistical Methods — K-Means Clustering

[2]https://qr.ae/pNvLYO

[3]https://predictivehacks.com/k-means-elbow-method-code-for-python/

[4]http://www.datanovia.com/en/wp-content/uploads/dn-tutorials/004-cluster-validation/figures/