Here's a couple of resources for you:

https://next.nostr.watch/ - This site lets you explore all publicly accessible relays. Currently about 1200 relays.

https://stats.nostr.band/ - This is an interesting Nostr stats page for a lot of different data-points. Bear in mind, though, it only shows information that nostr.band knows where to gather from. Any portions of the network that are not accessible to it won't be included in the stats. Toward the bottom of the page you can see which relays are being used the most.

https://jumble.social - This is a Nostr web client that puts relays front and center. If you go to the "Explore" page, you can see which relays your follows have favorited, if any, or a list of all relays Jumble can find. Selecting the relay will show you a feed of the notes that are on it, if it allows you to read from it.