That's a great question, I don't know. Maybe it can't be answered, because it's dependent on a whole bunch of social factors, like how likely relay runners are to censor users, or what a given user's risk profile for censorship looks like.

In theory, 2 relays for the entire network is enough. But that introduces scaling issues and the possibility for them to cooperate in banning someone. My best guess is that at the network's current size, 5-10 hubs is probably enough, plus relays that support special use cases, like authenticated communities and archival relays. Nerds will run their own relays, which is fine, but it doesn't really benefit anyone as long as censorship or relay failure is only occasional. I guess what's most important is that 1. there are backups of important data, and 2. it's easy to create a new relay when one is needed.