You can find good information about relays on nostr.watch or stats.nostr.band
You realistically only need a few relays. I am rolling with 6 right now, with only 2 paid relays.
The main benefit you get with paid relays, if you aren't also pulling from free relays for the same feeds, is greatly reduced spam, particularly if you ever go to the Global feed. I would ONLY have Global feed enabled for paid relays, if your client allows that kind of granular control.