Ronald1,
Here you have the answer:
Given the 51 boy name server names and the 50 girl name server names, we have 2,550 unique combinations. This has been more than enough to ensure that the correct DNS records are pushed to our network, even when two people attempt to sign up the same domain.
So there are a more than 500 sites that share the nameserver

. In 2012 (
https://news.netcraft.com/archives/2012/11/12/cloudflare-accelerates-235000-websites.html) there were almost 1 million sites using CF.