Not when you're using "in" I think. Not sure why you can't use the locative and drop the "in" though. It is consistent throughout the course for America, perhaps because it's not a Latin word?

Yeah I imagined it would be "Quot urbes Americae sunt? based on how "Roma" was inflected to "Romae" in other examples.

That's only for very few words, for cities, towns, and small islands, never for a large area.