Sarah,
I don't think that they can say how many people you have in the birth with you, unless there is a need for intervention but even then they will just ask everyone to leave.

I have been in births where there has been a room full.
One very sad birth I was involved in were two very young people, whose whole family ( from both sides) insisted on coming in and proceeded to argue over the poor girl who was in active labour about who was going to get custody of the baby! I had to ask them to leave and then I had to call security when they became abusive... my point is, unless there is a good reason to limit the number, midwives aren't able to dictate who can and can't be there.