I used https://nostr.watch/relays/find and just picked a few.

My personal rule is: 10 Relays.

3 Big relays (relay.damus.io, relay.snort.social, nos.wine)

5 randomly picked relays

2 specifically picked relays (one of which is my own in fact)

Sometimes I add one more, sometimes I remove one, depending how I feel. So I usually over around 10-12.

You follow people.

That said, there are clients that use the "Gossip model". See, many events (notes, likes, zaps, ...) have something called a "relay hint". This hint can be used by a supporting client to temporarily add it to your list of connected relays - thus expanding what you see.

So, depending on which relays you use, you either expand, or narrow, the amount of things you see. If, for instance, you enable the nos.lol relay, you are guaranteed to see a lot. But, another relay such as relayable.org, might have different things going on on it.

In the end, it is a bit of trial and error untill you have a list of relays you like the most - or more specifically, a list of relays that show you what you wish to see.

Remember, the basic principle of Nostr ("Notes and other stuff transmitted over relays"), is that when two people connect to the same relay, they see each other and can interact. Not everyone posts to all relays. So finding a list of relays that basically represents "the common demoninator" is what most people starting out do. If you are on Snort, you can go to my profile and click relays and see which ones I use, for instance. Then do the same on somebody else, and again. You will notice that between all of us, we might share one, maybe two relays.

Another thing to consider is that some relays broadcast and fetch notes from other relays directly - or, in laymans terms, "they synchronize". So whilst relay A and relay B might have different addresses, they might be syncing under the hood and thus are effectively the same in terms of data. :)