[yakubin]

I'm not on NixOS, but I do use Nix as a package manager on Debian and this is what I got:

  $ nix-env -qa | wc -l
  39168

Of course, it's important to note that Nix doesn't lag behind upstream as much as Debian. Actually, it's usually one of the first places where new versions of packages appear. So you don't have the problem such as the one Debian has with maintaining PHP 7.4.

[faho]

Raw package count doesn't really compare, considering that Debian splits up packages a lot.

For example they have a lot of "-dbg", "-devel" and "-doc" packages that other distros might just choose to leave in the main package instead.