Presumably qintrȳti is the genitive plural, rather than the dative or locative (they have the same form in the aquatic plural in the fifth declension).
The vocabulary list at dothraki.org confirms this as genitive:
int.pron. how many (+ genitive, usually plural) ( < skore + verdon.)
Skorverdon valoti ēdrusi? — How many men are sleeping?
Other examples exist:
Skorverdon azantoti riña urnes? which anyway rules out the locative, as azantoti (solar, 3rd declension) is only for genitive and dative. https://www.duolingo.com/comment/23530560
Skorverdon valoti dāri majaqsa? https://www.duolingo.com/comment/23604130
Skorverdon valoti ēdrus? https://www.duolingo.com/comment/23608027
Basic use: possessives, and in general anything that would be expressed with "of" in English.
My guess - we're meant to think of this as How many of the turtles did you see?