You will always have the option to go from a double to single occupancy and vice versa. The hostels have a capacity that exceeds the annual intake. As a result, once you arrive, this should not be an issue.

But If you are concerned about expenses, then take a double occupancy room and then shift your room among your friends. Or you can book a single occupancy room, that way you will have your own personal space than without any disturbance.