I'm not aware of the changes affecting the number of quarries required vs. the number of mines required? Forge isn't just for iron though - it's also for mines you build on regular hills, for the science, and for the engineer slot it provides. Plus it gets production if you have an arena in the same city
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If your goal is to increase your population, I would recommend building more cities anyway. Other options would be improving the food resources in your smaller cities, building granaries in them, and sending internal trade routes to them.
As for if it's intended to be that high, the pop requirement for all national wonders was increased by 5. Perhaps that's too much? I can't really say yet myself. For comparison, the base pop requirment (at 1 city) for National Epic is 22, for Scrivener's Office it's 20, and for National Monument it's 27.