KMO 2017 Problem 8

For a positive integer $n$ , there is a school with $2n$ people. For a set $X$ of students in this school, if any two students in $X$ know each other, we call $X$ well-formed. If the maximum number of students in a well-formed set is no more than $n$ , find the maximum number of well-formed set.

Here, an empty set and a set with one student is regarded as well-formed as well.