What is the Linux distro ranking based on number of packages?

Well, here you find some information, I don't know if accurate or not, I suspect not too much.

Anyway, each major Linux distribution has almost everything one can ever need. What is missing are essentially niche applications, or applications that in some way cannot be packaged.

This is in principle an objective question (count the number of packages available out there), but difficult to answer for lack of comparable figures.

If you count only packages that come with the distribution, then the table in Wikipedia (thanks to enzotib for the link) gives the answer. Or rather it goes some way towards an answer: different distributions break up packages in different ways. For example SuSE tends not towards monolithic packages, so it has barely more binary packages than source packages. Debian, which usually separates anything that can sometimes be installed separately, has about the same number of source packages, but twice as more binary packages. All in all, most major Linux distributions (Ubuntu, Debian, Arch Linux including AUR, OpenSuSE, Fedora, Mandriva) have a similar amount of software.

There is more difference if you count third-party repositories and binary packages provided directly by vendors. Simply put, the more users a distribution has, the more people are inclined to make packages for it. I don't have any hard figures, but I suspect that Ubuntu is a winner here, both due to its large collection of user repositories¹ and due to the widespread support (because of Ubuntu's popularity). For enterprise software, Red Hat is also a strong contender.

¹ _{Personal package archives (PPA) — entry point for users, entry point for developers.}