Tumblr is one website. Not 500,000. Even though 500,000 people have a blog there, it is still tumblr.com. Even if you do set up a subdomain such as my-blog.tumblr.com it still just redirects to tumblr.com/my-blog. Counting tumblr as 500,000 websites is like counting everyone’s Facebook profile as its own website.
The relevant number is 500,000,000, which is 1000x larger than the number you're citing. But also your comment about subdomains is wrong: redirecting to tumblr.com/my-blog is an optional setting, with the default being to use a subdomain. You can also use custom domains rather than a tumblr.com subdomain. Tumblr has had this functionality for almost its entire existence.
Yes, and the user I directly replied to above incorrectly said "500,000" three times, which is only 500 thousand. Tumblr exceeded that blog count well over 15 years ago.
Your original comment, about how a Tumblr migration would double the WP site count, was correct. That isn't what I replied to above.
I'm fairly confident that's not how WP.com does it. On WP.com - via WP multisite - each site has it's own instance of WP and therefore each counts as a site running WP.
Tumblr's injection would result in a significant increase in the number of websites running WP.
