There's about 360 million domains (according to Verisign as of March 31, includes inactive sites as well) and if Cloudflare manages about 1/3 to 1/2 of them, that's like 180 million domains at most.

700 changes per second is 1.8 billion changes per month. I guess 10 changes per top level domain per month is quite low, which suggests a very long tail distribution, which makes sense, given that most websites are just a static page a business sets up once, with DNS changes maybe once every few years.