Baby Crib and mattress!! Help please!!

Jyoanna • First time mommy ❤️! Pregnant with twin boys 👶🏽👶🏽 Baby Jayden And Baby Armani.. Due Date July 4th 2016 💙💙
My mother in law it's getting this crib for us! The problem is that i don't know how many mattresses she needs to get for this?? I don't have no idea how a baby crib works! I'm a first time mom, pregnant with twin boys 👶🏽💙 I'm due July 4th ☺️