I have a 10 gun extra deep that fits around 5 HFT sized rifles.
I have a 7-9 gun extra deep that fits around 3.5 similar sized rifles.
I have a 6 gun extra deep that fits a couple of shotguns and a fullbore.
I have a 3 gun that fits about 2 shotguns , or an air rifle broken down into pieces.
Buy the biggest you can afford would be my advice.
Shootingsafe.co.uk were the last company that I bought one from (only a few weeks back) and were a pleasure to deal with, highly recommended.