One is 2nd person singular (Hoşçakal), the other is 2nd person plural (Hoşçakalın). In other words if you say goodbye to a single person, you use the first one; whereas if you say goodbye to a group of people, the second version is to be used. Moreover you can use second version for politeness because just like French, we use that form in verbs, 2nd plural, instead of 2nd singular, to be more polite.