I have no source for 55 gal bags. 33 gal bags are std off the shelf. One point - the 33 gal bag can be worn at reg length as a poncho with 3 holes. A 55 gal would probably need a hem cut - but the added length could be very noce as a bivy bag.
It's still 3, sorry! It may not go up directly related to your number of squares. But it is definitely still three. I'm sorry that I don't know the system for how many you can give. That's the biggest mystery of the entire system.