I try to stay around 10 or less and go for popular relays that run strfry (nostr-rs is good too).

It’s also good to have a relay that does aggregation like nostr.band or nostr.wine (paid).

The nostr.wine filter relay also rebroadcasts your notes. IMO, the best client experience is using this set up with only 4-5 backup relays, but you need to pay for it.