If you write to just one relay, they should still all get the events. We need to figure out to charge for reading from relays, not only for writing to them, which all the paid relays are about so far.
Once the marginal cost of each extra user turns into a marginal profit, people will figure out how to serve a million users with "one" relay.
quoting nevent1q…w0adThere should be a few thousand relays (good ones) a least a few in every country in the planet and not hosted in the great cloud
Clients is a bit ambiguous because there will be apps which have different purposes
Nostr band can already give some idea of distribution based on the events published per relay, Damus seems to have a huge influence on this since they use a static list of recommended relays
Ideally each user picks their own relays where they want to write their events and everybody should use Gossip approach to get those events of they are following them
If 1M people follow me then I need to write to a few relays so that those relays can handle that load
And clients should use the relays that are responsive in that set