"How many chairs do you want?"
Translation:Kolik chcete židlí?
This is plural genitive.
Genitive is used for objects of many verbs and with a few prepositions, and also occurs in constructions with nouns/noun phrases (often to show ownership), adverbs of quantity, and most numbers. Think of this as the equivalent of the English expressions with "of", such as "the name of her husband", "the color of your eyes", and "a lot of water".