How to classify features into two classes without labels?
As you know, clustering is Unsupervised learning algorithm. Since you don't know the number of clusters, it becomes hard to find the best possible separation (number of clusters). There is a very good paper published on validating clustering techniques. There are 3 criteria defined in this paper for validating your clusters. You can take a look at that.

Also, the link below gives you some code in R for cluster validation. If you want you can try this too-

http://www.sthda.com/english/articles/29-cluster-validation-essentials/97-cluster-validation-statistics-must-know-methods/