Clipper [1] alone has over a million installations. Others as you say have 500k, it all amounts to millions of users of clipboard managers specifically labelled as such. Add other applications which couldn't work without a clipboard monitoring feature ( KDE Connect has been mentioned already, translation applications, text and code editors, the list goes on... ) and you jump to tens of millions if not hundreds of millions.

[1] https://play.google.com/store/apps/details?id=org.rojekti.cl...