i guess we don't really know and that's kind of the beauty of it, because entire segments of the network could be hidden behind the Great Firewall of China, for example, and we'd never know. there are also certain relays like Japanese relays, that only allow Japanese IP addresses to connect.

but if you check out https://nostr.watch that will give you a good idea on the size of the network and probably most of the relays that are available.