It's not the number of websites hosted on a server that matters as such, but more about the amount of resources each of those websites are using. You could host 10,000 sites running simple HTML sites with no problems, but add MySQL to the mix and that's where the problems start.
You could have just one website on a Shared server that if has too many MySQL queries, due to let's say, poorly written sloppy code in a script, then that one website hosting that script alone could bring the server to a halt.
No need to work out which providers are best to use going by how many domains they use; rather just make sure the providers use CloudLinux (OS/software used to help prevent users abusing resources), and well, just managing the server correctly basically.