As others have said, divide by two and that is what the safe will hold in reality. Keep in mind this is just a rule of thumb though. I actually have an AMSEC 6030 so let me tell you what I found.
The 12 means:
The safe will hold 12 if you use the left side of the safe for long guns and you fill the right side with shelves.
All my rifles have scopes so I can fit 6 on the left side comfortably.
The 18 means:
The safe will hold 18 if you use the right side of the safe for long guns and you fill the left side with shelves.
Again my rifles have scopes so I can fit 10 on the right side comfortably.
The 30 means:
The safe will hold 12 long guns on the left side and 18 long guns on the right side. Off course you won't have any shelves except for the one on top in this configuration.
So in my case if I removed all the shelves I could comfortably hold 16 long guns.
I will also mention that with the AMSEC 6030 on the right hand side, and maybe other safes, you can't take out a long gun unless you get the row of guns that are in front of it out of the way first.
And as a relatively new safe owner I can't stress, BUY A BIGGER SAFE THAN YOU THINK YOU NEED. The safe will service you for generations. It's a matter of when not if you or your family will outgrow it. I did not take this advice very seriously.
I was going to buy a 6032 but decided to go a little bigger and got a 6030. Now a few years later I should have bought at least a 7240.
That's my two cents.