As I said, it is a statistic from this linked study above:
Adjunct Faculty Demographics and Statistics [2023]: Number Of Adjunct Facultys In The US (zippia.com)
I want to add that all these numbers relate to adjunct faculty and don’t take into account the fact that much of the teaching at some schools is done by grad students, either as TA’s or instructors. There have been strikes for grad student pay and benefits as well. Their labor is dirt cheap and stipends tend to be low.