According to The Hosting Tribunal there were over 500 million blogs in November 2019. Tumblr reports over 440 million and WordPress claims over 60 million and there are several other big providers and tons of little ones.
We have a massive database of a little from here and a little from there but some from of all of them. We submit to 500+/- blogs everyday and never submit to the same one more once or twice a year. We do it that way to provide our clients with fresh blogs to send to every day.