Oh, FINE, I'll zoom in on panel five! Twist my arm, why don't you.
So yeah. That's over a thousand seers. When Magic (or whatever) said very few wizards / seers have any idea what they are, it wasn't kidding. Had those other two magic using seers been eligible, about 0.5% of all seers on the planet would have been represented.
Arthur, having basically only what's been said in this scene to go on in reagrd to seer population, was expecting a significantly smaller number.
Why Magic Changed
The "why magic changed" refers to the overexposure of magic and people being on the verge of figuring out how to make it commonplace. It is not in reference to magic asking seers for their perspective.
In other words, if magic did change, and if magic was considering changing again in a few months, there could potentially be around a thousand seers present for that second consultation (hey, a new record!) The second purpose would not have been automatically revealed to them earlier.
It's not until you get to a third potential change within a single generation that there's a potential "every seer who's used magic knows by default" conundrum.