I used https://nostr.watch/relays/find and just picked a few.

My personal rule is: 10 Relays.

3 Big relays (relay.damus.io, relay.snort.social, nos.wine)

5 randomly picked relays

2 specifically picked relays (one of which is my own in fact)

Sometimes I add one more, sometimes I remove one, depending how I feel. So I usually over around 10-12.