Think about it this way:
The architectural necessity for any one person to host a relay is not directly proportional to the number of users. That is why we don't need to put a relay in every client and therefore on every device (this would mean one person might end up with 3+ relays, which is complete overkill).
If the network has 2 users, it'd be best to have at least 2 relays.
The same, however, is probably also true at 10 users and 100 users.
At 1000 users, you might want to add a 3rd relay.
At 10000 users, 10 relays might be good.
And so on.
