This question comes up a lot, and for good reason. It's an important detail, since the last thing you want is to fill your gun safe up too quick and have to get another one. So, here's some insight to help you figure out what safe you need.
How Many Long Guns Will My Gun Safe Hold?
A Specific Design
A reputable gun safe brand is going to give you a listed gun storage count, but they're going to call it "long gun capacity", or they'll say, "Up to [this many] long guns". There's a reason for this phrasing. If you count out the notches they give you for long guns, it will equal what they are listing, but looking at the spacing of the notches, they tend to be a little on the small side. Most of these single spaces won't be big enough for scopes, bipods or other modifications. Also, rifles and shotguns with large stocks can have crowding issues on the bottom.
These companies aren't trying to pull a fast one on you, there's a reason for this design. The racks are designed with smaller long guns in mind so you can store more of the smaller firearms and use multiple spaces for larger or more bulky long guns if needed.
Here's an example. Let's say someone has a gun safe with two racks, each with 15 spaces, one of the left and one on the right. The owner has eleven .22 rifles with small stocks and two long rifles with scopes attached. Even using two spaces each for his scoped rifles, he's still likely able to get all of his long guns on just one side, and still have the other side of the safe for other things like ammo, documents, etc.
Designed to get you the most out of your space.
Now, imagine if the same safe had gun racks with only twelve spaces. The spaces are larger, but there are less of them. The smaller guns will have a little more room, but he won't be able to store his whole collection using one gun rack. He'll have to store the remaining guns on the other side, losing all that storage for other things.
So, what does this mean for you? How can you figure out if a gun safe will hold all of your guns? The simple way to think about it is like this: if a long gun has a large profile (scope, wide stock, etc.), count it as two. If you want an even simpler approach, you can say you'll get about two-thirds of what the safe's capacity is, but this is kind of an overly simple way to look at it.
One last point. You'll hear gun safe dealers say this all the time: "Go bigger than what you think you need". Over time you might collect more firearms, or modify the long guns you already have. Going bigger means you have the space to grow and you'll be less likely to come back in a couple of years to buy another safe. Going bigger now will save you the hassle later.
Knowing what "Long Gun Capacity" really means can help you figure out which safe can hold your total long gun collection before you buy, and this can help you avoid buying a safe that's not going to hold what you thought it would. Whichever safe you decide on, make sure its going to do what you need it to do. It'll save you time, money and frustration in the end.