Setting 2-3 for each category would be ideal.
Since the outbox refactor you'll see amethyst connecting to 100s of relays but that's automatic. It's the relays that other nostr users have set in their profile.
Discussion
Im using Primal and Damus for mobile — currently I added 15 relays from a Reddit post that had a list of relays but now I’m wondering if adding so many at once is overkill haha