And really, even "there are 10 windows in the building" is not something you'd normally say. It's "The building HAS 10 windows." I get that Hungarian doesn't use the possessive that way, but it feels odd having to remember strange phrases in English just to satisfy the parser.
Is this for the same reason that it is "ablak" instead of "ablakok"? The question word "Hany" or "How many" must function as a quantity indicator which causes the object to be handled in the singular rather than the plural. Is this so?