Depends. There are no, to my knowledge, hard numbers since anyone can download and install Linux as many times as they want.
Beginners and experienced alike use Ubuntu. Especially if they don't want to switch between home workstations and servers or workstations at work.
Debian is popular with admins because it tends to be straightforward and no nonsense due to their strict guidelines of what is and isn't endorsed by their philosophy of software. Usually if you want a specific purpose server, Debian is very stable and doesn't include a lot of stuff you don't want.
Red Hat's offerings are commercial and offer good support. Hardware compat lists, phone support, etc. are good for businesses.
Underneath it all the Linux kernel is the Linux kernel. The only big difference comes from the default and extras installed and how loose a distro plays with security updates.
You should install what you're most comfortable administrating. If you are used to the tools in Ubuntu, go with Ubuntu. Or Debian. Or CentOS. Whatever floats your boat. You're going to be better off using what you're comfortable and knowledgeable with than shoehorning into something you're not familiar with and in some cases subtle differences can create some not so amusing results (why would issuing that killall command make the server shut down?...oh...)