Ok, so i have two answers for you and they are both true at the same time.
1:
Thats the nice thing, it does/should not really matter. What the mechanism is, is that you declare at what relays you put your own stuff. If i follow you, i am aware of that declaration (obviously this is a list thats there in the background and its all automated etc. etc.), so i just go look at those relays and ask for your stuff. These are your ''write, or outbox'' relays. At the same time, you declare what relays you keep an eye on, such that if i post something i want you to be aware of (like this comment on your post for example), i make sure to (also) send it to those relays such that you get a notification. These are your ''read, or inbox'' relays.
It does not really matter all that much what relays you use (other than how fast and reliable they are, how long they retain/store events and are generally good actors that dont censor you). Generally you could say having 2 dedicated write relays, 2 read relays and 1 read&write relay (so 5 total), should have you covered to the most part. Consider getting 1 paid relay. Then there are also DM-relays, similar story, could be the same relays, could be different more dedicated relays for that purpose.
All of this is the stuff you set up in your relay setting and constitutes your relay list. From this perspective, being on more relays does not really make a lot of sense, and will only slow things down and make clients sluggish because they will be needlessly connecting to more relays.
2:
Outside of your relay list, where the specific relay does not really matter that much and they are interchangeable. Relays can also function as this dedicated place/platform/website, that is moderated (be that based on people, or content, or whatever metric you can imagine) for a particular purpose. Those relays are more like websites you visit and browse and see whats up. The moment you see someone interesting on one of those relays and follow them, immediately the dynamic described under 1 applies as well. Yakihonne is a phone/web client that allows you to browse those relays, or have a look at jumble.social that also makes browsing relays, for example this one: https://jumble.social/?r=relays.land/spatianostra .
I made this silly little video about it, that i am now reminded off that i need to repost on Nostr the correct way, but il just use the youtube link for now.
https://www.youtube.com/watch?v=CL34LnBs0OQ
PS: Im an asshole with a terrible relay list that i still need to clean up, so don't feel bad in anyway, i suck at this myself, just do as i say don't do as i do and all will be good 😉
