73 million sites comes from our friends at Meanpath (their search engine isn't running anymore, sadly). They showed us on 7.3% of websites. Best guess is there are about a billion sites on the internet. Thus, 73 million.
Interestingly the number of active sites hasn't materially changed in 5 years, but host names have skyrocketed.
[0] https://news.netcraft.com/archives/category/web-server-surve...