Hard to say, depends on your desired nostr experience and bandwidth limitations.

Somewhere in the 4-6 range has been working well for me (but that includes filter so the reach is much larger). I used to have more paid relays but removed many of them because they were slowing me down. Have to experiment and find a mix of performance/reach/content that works for you. It’s complicated and unsolved but it’s what nostr is all about.