Hola Cihan

For other users to understand as they may find your question useful, here's the translation (from google translate) to your question from Turkish into English:

"At the beginning of the sentence it says unos amigos. Shouldn't un be amigos? So at a friend's house"

As the text in Spanish is saying "Estoy en casa de unos amigos...", literally "I am at the home of some friends", we're using the plural "unos" - if we said "I am at a friend's house" then you'd see the article in singular "Estoy en casa de un amigo". 

I hope this clarified it.

Saludos cordiales