When reply to an event is posted most clients include the source relay in the reply (relay of event to which you're replying). So by reading from one relay you eventually learn about other relays, when users interact with them. That's what my server does. As soon as it discovers a new relay it starts reading from it and expands it's network. Right now the number of active relays I read from is 1126 (at the bottom of https://stats.nostr.band/).

But, it doesn't mean that I will _serve_ all events from all relays to clients. 70% of all events published on big public relays is spam, which I filter. Sometimes there might be false-positives. So it's probably not about 'which relays I'm reading from', but 'was that event spam filtered'.