The answer to this is not entirely clear - Questing Country is so vast that it's fully possible for two Dreamers to go decades without meeting, and so it's hard to get a grip on just how many there are. Certainly fewer than one in a thousand people in the Known Worlds is a dreamer, but probably more than one in a hundred thousand?
The number of Dreamers gathered in the Hushabye Meadow is certainly not anything like all Dreamers everywhere - it's as many as the Oracles near the site of the merge could gather together on short notice. One can safely assume that everyone knows several Dreamers not present, but as their dream-missions were in some sense near one another, it's also plausible to know most of the Dreamers from one's own era, especially as a long-lived Oracle.
Hopefully this helped answer the question despite not being able to give a specific figure.
The influence of the senpai is meant to be strictly advisory - these are the best-informed individuals on-site, and useful as experts, but not as authorities.
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The number of Dreamers gathered in the Hushabye Meadow is certainly not anything like all Dreamers everywhere - it's as many as the Oracles near the site of the merge could gather together on short notice. One can safely assume that everyone knows several Dreamers not present, but as their dream-missions were in some sense near one another, it's also plausible to know most of the Dreamers from one's own era, especially as a long-lived Oracle.
Hopefully this helped answer the question despite not being able to give a specific figure.
The influence of the senpai is meant to be strictly advisory - these are the best-informed individuals on-site, and useful as experts, but not as authorities.