"Number of packages" is kinda hard to define. A good approximation can be done by searching for the relevant arch name on the latest nixpkgs eval on Hydra and looking at "Still succeeding jobs" + "Newly succeeding jobs". Here's the links for aarch64-linux (61481 total) and x86_64-linux (72227 total) for example. I don't think there's a way to easily get historical data, you probably have to write a script that queries hydra or something.
Yep! I've been running NixOS on Librem 5, which is aarch64, and everything I needed was available straight from the cache, which was nice.