ZorinGroup wanted the same answer to that question; Therefor Zorin OS comes with Zorin Census pre-installed.
Zorin Census does not track, read, record or report any data on your computer. The only function it performs is to ping if used. As open source, this is (and has been) independently checked and any user can do the same.
As far as how many users- I heard a rumor that it was around 1 million active Zorin OS users. How accurate that rumor is, I do not know.