> Is there a way to see which notes are on which relay ?
If you want to share a note, you can share it as nevent bech32, and this can contains in which relays you can find this note.
If you mean user notes, you may found their relays list in one of your relays if they intract with someone in it, and you can get their notes from their outboxes relays.
If you want to share your public key, you have to share it as nprofile bech32, because your relays can be in it.
> we should only have around 2 - 4 relays ??
You can have as much as you want, but the recommended number is 4
You can use nak.nostr.com to encode/decode bech32.
If you want to know more about bech32-encoded entities https://github.com/nostr-protocol/nips/blob/master/19.md