I see, but in Hungarian it is not usual to use plural? Houses have normally more than one window.
You can use the plural but it is not necessary. Without an article here it can be any number of windows, one or more.
Would you agree, though, that Duolingo should also accept "Every house has a window" here?
Not necessarily. Your statement, as I read it, says that there is a window at a specific spot on every house.
How do you explicitly express a possessed entity's quantity (i.e. plural/singular) in hungarian?
Like, if you have multiple of a thing? You have an -i- somewhere in the suffix:
- a házam - a házaim (my house - my houses)
- a házad - a házaid
- a háza - a házai
- a házunk - a házaink
- a házatok - a házaitok
- a házuk - a házaik
Basically you take the 3rd-person singular possessive form (háza), add an -i, and then add the proper possessive suffix. If the noun already ends on a vowel, though, you don't add the 3rd-person suffix, just -i: kutya -> kutyái (his dogs) -> kutyáid (your dogs).
There's a whole lesson about plural possessives later, but this is the brief version. :)
Can't we say :
all houses have windows ?
Although I recognize that it is haznak and not hazaknak