Debian also has a repository model which allows the packages to be served by a static file server, because the dependency resolution is entirely client-side. Npm doesn't.
Partially self-inflicted since a normal JS project will redownload the same packages over and over again, hundreds of times throughout their existence.
Number of packages:
Debian has 172,000 packages for the most popular architecture, amd64. i386 has 24,000 packages. The rest have less than 500 each.
npm recently broke a million packages.
Download counts:
I couldn't find numbers for Debian. npm served 11.2 billion downloads last week.
(Debian numbers retrieved by https://popcon.debian.org/, npm numbers from https://medium.com/npm-inc/npm-weekly-200-dont-miss-today-s-... and https://api.npmjs.org/downloads/point/last-week