So if 5 guests want to book and use four rooms they would have to make a reservation for 7 guests. Seems clear enough. But, it could be rather confusing to some people who fail to read or comprehend the room requirements and just focus on the number of people in their party, which is how Airbnb reservations are designed.
How about: Pricing is per Double Bedroom. Four Double Bedrooms available : 2 guests max. per room.
Example: if you are three people who each want your own separate bedroom you need to book for six.
Giving examples often works better than asking people to figure it out for themselves. But, whichever way you do there will always be someone who fails to understand. I would send a message as soon as a guest inquires or books and point this out.
From your example "5-6 guests get the use of 3 bedrooms" would be clearer to say 3 double bedrooms accomodate a minimum of 3 single guests or a maximum of 6 guests sharing a room at 2 guests max. per room.