According to the Wikipedia comparison tables, Ubuntu has ~47K available packages, while other major distros, under this particular aspect, have a smaller number.
I.E. openSUSE has ~40K, Debian ~37K, Fedora ~22K, ArchLinux ~10K and Chakra ~3K (yes, I know, Chakra is not a major one but I'm keeping an eye on it because I find interesting its purpose of being KDE-centered).
Why these great differences? Compared to other distributions, Ubuntu is a fairly young one, I can't understand how it has a package availability greater than 20/50% against other distros.
Right now I'm using Kubuntu 12.04, but I'm also looking around for some alternatives for when I will have to upgrade it (in particular I'm interested in a semi-rolling distro, hence my attention to Chakra), and these numbers make me wonder if the software availability is more or less the same for these distributions.