The wording right now says that MediaWiki "is used by thousands of websites and companies." "Thousands" is a gross understatement, I think; probably Wikia/Fandom alone has over 1,000 wikis. It's hard to get an accurate count because of course a lot of instances are private; and there's the question of how active a MediaWiki installation has to be to count as being "used". Still, even by the most conservative count, I think the number has to be in the tens of thousands, maybe even over 100,000. As to the number of companies/organizations using it, thousands is probably right.
Topic on Talk:MediaWiki/Homepage improvements 2018/Proposal
Do we have actual stats instead of guesses? AFAIK the best numbers are in https://pingback.wmflabs.org/ and in https://wikiapiary.com/
Oh, I forgot about that "pingback" site. Well, there you go - the number is somewhere between 25,000 and 120,000, apparently.
I'm wondering how to keep that number readable to the human eye. Is the proposal to replace the word "thousands" by "tens of thousands"?
Yes - well, "tens of thousands of websites and thousands of companies" makes the most sense. Or, even better, "tens of thousands of websites, and thousands of companies and organizations".