Learn Turkish: Masada iki zar var, hangisi senin? - There are two dice on the table, which one is yours?

Hangi just means "which" and is used directly before a noun (like hangi zar - which die). By adding the third-person possessive suffix -si, it becomes a pronoun meaning "which of them" or "which one". It lets you drop the noun and refer back to the specific group we just mentioned: the two dice.

Since there are two dice, why isn't it plural like iki zarlar?

In Turkish, whenever you put a specific number or a quantity word in front of a noun, the noun itself must remain singular. So "two dice" is always iki zar, and never iki zarlar.

Why does the sentence end with senin (your) instead of a specific word for "yours"?

Senin is the genitive form, meaning "your" or "of you". To ask "which one is yours?", Turkish often uses this genitive pronoun as the main idea of the sentence (literally: "which of them is of you?"). You could also use the pronoun seninki ("the one that belongs to you"), but just using senin is very natural and common here.