https://blog.thesamecat.io/how-many-relays-should-i-have/
https://nostr.watch/relays/find
Choose relays that are fast (nostr.watch) and that have people in common with you. Mixing in a few paid relays can help with spam, and generally retain posts longer. https://nostr.wine is a good one. I would shoot for about 10-15 relays tops, especially if you are on mobile, and not able to charge your phone just whenever.