Welcome to the world of crabbing!!
It's so exciting getting your first crabs, but there are a few things you need to consider before you can get the crabs:
The first topic you have already touched on, this is the size of your tank. As YYWW said, you don't count the number of crabs you have by the size of the tank you have, it's all about what you have in the tank. You have the underground layer (underneath the surface of the substrate), the surface layer (the surface of the substrate that they can walk on) and what I call the second layer (this includes everything they can climb on). The amount of crabs you can have in the tank depends on how much of each you have. You need a lot of substrate for them to bury in either for fun or for protection. The surface layer isn't as important because crabs often don't spend a lot of time on the surface. Really, you just need enough surface layer so that they can walk to the essential places of the tank, for most tanks this would include food, water and shelter. And for the second layer, this you need quite a bit of. Hermit crabs LOVE climbing, if you don't have enough stuff for them to climb on, they're going to get depressed/stressed and that can be very bad for the crabs. In order of importance, I'd say that the underground layer is the most important. They're going to be underground a lot, mostly for fun but when a hermit crab molts, it has to completely bury itself in the substrate for two reason; protection from the elements and even other crabs and it's the complete darkness that triggers the molting hormone. The rule of thumb is 6 inches is the bare minimum, but when you get bigger crabs, it's about 3-4x the height of your biggest crab. The second most important is the second layer, because as I said, hermit crabs LOVE to climb and if you don't have enough for them to do, it can be really bad for you crabs.
The second topic isn't as important as the first, but it relates. This is about positioning the tank. There are a few rules of where you want the tank to be; you don't want the tank to be in direct sunlight, you don't want the tank to be in the direct path of an air conditioner because they need the heat and humidity and the third rule is that you need to have enough power points, but that can always be fixed with a power board. Other than that, remember you need to put it on something that would be able to support the weight of the tank and also remember that the substrate does smell a little, not much but if you're very sensitive about it then you have to be aware about it.
The third topic is about what to put in the tank and what to give them. The type of substrate you use is very important, you can experiment with this but the most common type of substrate would be Eco Earth or something similar. It's sort of like a dirt substrate that is very good for the crabs. The other option is to use play sand that you can find at your local Home Depo, make sure that there are no chemicals that have been used with it and wash it before you use it just in case. You can even use a mix of both, I'm going to start experimenting with 2/3 Eco Earth and 1/3 Play Sand. The next thing is you have to make sure you have the right water. Crabs require water, they can go for weeks without food (please don't test that) but they can only go a few days without water (please don't test that either). They require two different water types, fresh and salt. The fresh water you can get by buying some Tap Water Conditioner, aka Chlorine and Hard Metal Neutralizer. Follow the directions on the label, such as the one I use is 2 drops per 34oz. The salt water you can get by first treating the tap water with the Tap Water Conditioner and then dissolving special Hermit Crab Salts into it, remember to follow the directions on the label, mine says dissolve 1 teaspoon of salt into every 34oz of treated water. The third topic within this paragraph is going to be food. DO NOT USE FOOD BOUGHT FROM THE STORE!!!!!
This is probably one of the biggest things that new hermit crab owners do wrong. They go to a pet store, see some specially made hermit crab food and give it to them as their full diet. These hermit crab food sometimes contain chemicals which are harmful for the hermit crabs. If you feel like you're not giving your hermit crabs enough of the required vitamins and minerals, look at the list of acceptable hermit crab food, you can find that in the Food and Water section of the forum.
The fourth topic I'd like to talk about it heating and humidity. There are lots of different theories about how to properly heat the tank, my advice is do whatever works. For me I have a heating mat attacked to the side of the tank as well as using a light coming from the top. You want the temperature to be about 75-80 and the humidity to be about 75-80%. Try out different things and see what works for you. DO NOT PUT THE HEATING MAT UNDER THE TANK!!!!!
Even if it says Under The Tank Heating Mat. The reason for this is because when your hermit crabs dig down to molt, you don't want it to be too hot for them and force them to disrupt their molting to get to the surface and cool down. I found that the lighting hitting the water bowels makes the water evaporate and helps keep the humidity. The type of substrate you have also effects your humidity. The reason why Eco Earth is so popular between hermit crab owners is because, if you keep it moist, it helps keep the humidity really well. Whereas with sand, it's harder to keep it moist and it doesn't really help with the humidity. Although do not take this as a reason to ditch sand completely, there are plenty of crabbers who have only used sand and have no problems with their tank.
The fifth topic of this post (it's actually turning into more of an essay
) is the types of shells. This isn't really all that complicated, you just need to know what breed of crab you're getting. The most common breed of crab to find is the Purple Pincher crab, you can tell because it has one bigger claw that is a nice purple color. From what I know, Purple Pincher crabs usually go for the turbo shells with circle openings. I know that this might not mean much to you at the moment, so here's an example: http://www.hermitcrabpatch.com/Pearl-Go ... 135-04.htm
This doesn't really show the shape of the shell, but you get to see the nice big circle opening it has. There are lots of other different shells, find the ones that your crabs like. If you need help with this, look up the different types of shells and check what your crab is currently living on when you get it so you know what it will probably move into next time it needs/wants to. The website that I linked you to is actually very well known between crabbers and a lot of people buy what they have through it. DO NOT BUY PAINTED SHELLS!!!!
There are few reasons for this, but the main two reasons are that hermit crabs tend to eat little bits of their shells for calcium and the paint can be toxic. The other reason is that the paint might make the shell a bit sticky, and if the hermit crab gets stuck in the shell it can mean a very slow and painful death for the crab, something you definitely don't want.
I'm sorry for the long essay but I hope I covered everything you need to know about caring for hermit crabs. Don't forget to hold your hermit crab regularly so that they get to know you. If you have any more question, feel free to ask. As for buying hermit crabs at the pets store, I don't advise it buts it's a start. If you really need to then buy them there, I had to because I live down here in Australia and most of the hermit crab websites are based in America and shipping costs about $40.