[evanelias]

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.

[chiefalchemist]

Half a billion is 500 million.

[evanelias]

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.