Hi ! Sorry if this is a stupid question, but is there a way to follow a band? I'd love to keep track of the bands I like and there are so many!!

There is! On band, album and artist pages you'll see a tiny box with a star in it. It's right below the title of the page. Click that box and the page is bookmarked.

You can access your bookmarks here: https://www.metal-archives.com/bookmark

Is there a way to get a notification when you log in if one of your band's pages as been modified or updated?

I don't think there is, but you can sort the bookmark list by "last modified"