The number of children that can share a room depends on the size of the room and the nature of the relationships between the children. If one particular child is acting out, then it might not be a wise idea to put that child the same bedroom as other children.
Younger children are more compatible with sharing a room because they may be afraid of sleeping alone. According to Baby Center, twins often room together because they have so much in common. Once children reach school age, they may not want to share a bedroom with a sibling because of the need for independence.