If the outbox model is implemented correctly, every user will typically only read from one relay, and will write to a small handful.
Of course, reads/writes will change depending on the event the user is interacting with.
Discussion
Users will probably read from a relay that uses negentropy or algorithm DVMs to aggregate events from other relays. Anyone who needs to tag, reply to, or DM that user will write directly to that aggregation relay.
Most apps will probably send writes to app- or community-specific relays. Boosts, negentropy sync, replies, and tags will gradually disseminate events across the network, making Nostr a distributed, eventually-consistent event store.
😅🤷
Eventually we'll converge on my thesis, that each npub should be synonymous with a running relay. A peer to peer Nostr (with central aggregation relays still possible, but a degenerate case)