There is no right way. Look at who you follow and add relays they have to your list so you can interact with them and vise versa when they post notes.

There’s no 1-size-fits-all solution. All depends on your device, bandwidth and your expectations. Generally 8-15 is what fits most. There are some relays which read info from multiple sources and filter it, thus taking the load off your device, such as nostr.wine and nostrich.land. My guide elaborates on that: https://lnshort.it/nostr-relays

15 is a good number. Let me look at which ones you're using.