There are no instances in AT, there is just one big server ran by the Bluesky Corporation. They used to call it "BGS" (Big Graph Server) but now they call it "Relay".

The Bluesky marketing is great at confusing people by giving them a false impression of decentralization: they let you run your own personal datastore with just your posts, and then the BGS will fetch your posts from that, no one else will connect to it, but that is just an unnecessary intermediary between you and the BGS, you could just publish directly to the BGS and I don't see why you would not do it.