How many packages are in the main repository?

You can run the following command line:

for f in /var/lib/apt/lists/*Packages; do 
  printf '%5d %s\n' $(grep '^Package: ' "$f" | wc -l) ${f##*/}
done | sort -n

that on my machine show the following output

27433 it.archive.ubuntu.com_ubuntu_dists_oneiric_universe_binary-i386_Packages
 7768 it.archive.ubuntu.com_ubuntu_dists_oneiric_main_binary-i386_Packages
 1944 it.archive.ubuntu.com_ubuntu_dists_oneiric-updates_main_binary-i386_Packages
  664 it.archive.ubuntu.com_ubuntu_dists_oneiric_multiverse_binary-i386_Packages
  632 security.ubuntu.com_ubuntu_dists_oneiric-security_main_binary-i386_Packages
  605 it.archive.ubuntu.com_ubuntu_dists_oneiric-updates_universe_binary-i386_Packages
  167 security.ubuntu.com_ubuntu_dists_oneiric-security_universe_binary-i386_Packages
   67 it.archive.ubuntu.com_ubuntu_dists_oneiric-backports_universe_binary-i386_Packages
   24 it.archive.ubuntu.com_ubuntu_dists_oneiric-updates_multiverse_binary-i386_Packages
   21 it.archive.ubuntu.com_ubuntu_dists_oneiric_restricted_binary-i386_Packages
   13 extras.ubuntu.com_ubuntu_dists_oneiric_main_binary-i386_Packages
   12 archive.canonical.com_ubuntu_dists_oneiric_partner_binary-i386_Packages
   11 it.archive.ubuntu.com_ubuntu_dists_oneiric-backports_main_binary-i386_Packages
   10 security.ubuntu.com_ubuntu_dists_oneiric-security_multiverse_binary-i386_Packages
    6 it.archive.ubuntu.com_ubuntu_dists_oneiric-updates_restricted_binary-i386_Packages
    0 security.ubuntu.com_ubuntu_dists_oneiric-security_restricted_binary-i386_Packages
    0 it.archive.ubuntu.com_ubuntu_dists_oneiric-backports_restricted_binary-i386_Packages
    0 it.archive.ubuntu.com_ubuntu_dists_oneiric-backports_multiverse_binary-i386_Packages

Each distribution is different. But you can get a good idea of what is available by using "aptitude" and looking at the numbers.

as an example (not installed packages 32180)

You can check it here on repology.org.