First and foremost, get him a friend!!! Hermies are pretty cheap at most pet stores, but they do not do well on their own. They will literally die from loneliness without socialization.
I have heard anywhere from 10 to 15 gallons per crab, I keep my 4 in an Exo-Terra 24x18x18 tank with front opening doors and a mesh top. The mesh top means I do have to mist them once a day, but I prefer it to a fully enclosed model. Sadly I couldn't find any on Amazon. By the way, the rock formation in the back of the tank is just a styrofoam insert, which I removed before decorating. Mine is admittedly a bit smaller than I would like, but they have enough room to dig and explore their environment, plus they can escape each other if they need alone time.
Basically I filled the bottom with a mix of coconut fiber and plain old play sand (1:5 or 1:6 ratio depending on your ambient humidity and how well the sand binds to itself) and wet down the entire thing until the sand is damp and holds its shape so they can burrow but not so much that it is sloppy or muddy. Don't want to drown the crabs!! I slapped a medium-size warming mat on one side of the tank (outside) below the substrate so they had a warm spot and can thermo regulate and filled the inside with things to keep them busy. Make sure they have room to dig!! Minimum depth should be either 6" or the height of your tallest crab plus 2". Then create a slow grade slope to the other end of your tank up to about 10"-12".
Lots of balsa wood and other branches to climb over, plenty of hides (coconut halves, hollow logs, fake rocks) flat bottom dishes (which they enjoy burrowing under) and fake plants. I have two small dishes for food, one for a basic organic hermit crab diet that I supplement with dried brine shrimp and another for fresh fruits and veggies, both for which are cleaned and replaced each day.
I also have two larger dishes that I use for wading pools. One is filled with fresh water and the other with salt water. Hermit crabs require both, as they drink the fresh water but require salt/brackish water for bathing. Again, refill daily. Mine enjoy throwing sand in them when they dig, but it's simple enough to just flip them over, rinse them out and refill. I use Instant Ocean, but mix any marine salt according to package instructions. Make sure that they are shallow and easy for the crabs to climb in and out of, though, as they will accidentally drown if they can't get out of a pool that is too deep.
I also make sure that they have cuttlebone available for additional calcium as well as spare shells of various sizes and I sprinkle in a few mini catappa leaves, which are rich in tannins and have anti fungal and antibacterial properties. Without a good source of calcium, desperate crabs will often cannibalize or steal each other's exoskeletons during the molting process.
One last thing: always condition your crab's water!! The chlorine in tap water isn't good for them. I have a single bottle which I use for my crabs and all my reptiles. I simply mix a whole gallon at a time and keep it in a jug and I have barely put a dent in it, so it really is a sound investment.
Here are some links to the items I use:
Water conditioner (note it says for reptiles, but it's exactly the same as the conditioner for hermit crabs): Zoo Med ReptiSafe Water... https://www.amazon.com/dp/B0002568SG?ref=ppx_pop_mob_ap_share
Hermit crab food: All Natural Hermit Crab Food by... https://www.amazon.com/dp/B01FJ6BVVG?ref=ppx_pop_mob_ap_share
Cholla wood branches: SunGrow Natural Cholla Wood, 3... https://www.amazon.com/dp/B01FJFFY6K?ref=ppx_pop_mob_ap_share
Zoo Med thermometer and humidity gauge: Zoo Med Labs Digital Thermometer... https://www.amazon.com/dp/B06XY3X7P9?ref=ppx_pop_mob_ap_share
Coconut hide: Eco-friendly Hermit Crab hut -... https://www.amazon.com/dp/B01MR7XKXH?ref=ppx_pop_mob_ap_share
Small rock dishes: Fluker's Corner Rock Dish / Bowl... https://www.amazon.com/dp/B00NG0GMAI?ref=ppx_pop_mob_ap_share
Floating cuttlebone: Zoo Med Laboratories SZMTB1... https://www.amazon.com/dp/B001CCVLUS?ref=ppx_pop_mob_ap_share
Water dishes: Senzeal Trapezoid Reptile Water... https://www.amazon.com/dp/B075L8Y6RF?ref=ppx_pop_mob_ap_share
Catappa leaves: 50 Mini Catappa Leaves - Best way... https://www.amazon.com/dp/B01G78H5XS?ref=ppx_pop_mob_ap_share
Compressed coconut fiber: Zoo Med Eco Earth Compressed... https://www.amazon.com/dp/B00TNLIANQ?ref=ppx_pop_mob_ap_share
Play sand:Mighty Max Battery Rechargeable... https://www.amazon.com/dp/B00005C6E3?ref=ppx_pop_mob_ap_share
Instant Ocean sea salt: Instant Ocean Sea Salt for Marine... https://www.amazon.com/dp/B000255NKA?ref=ppx_pop_mob_ap_share
Please note: I am not a vet or a professional, just an avid hobbyist and these are just the things that work for me. My crabs are happy and healthy and have been since I've gotten them. If anyone has better information or advice please feel free to correct me, just don't be a jerk about it, please. :) I am happy to accept that there are different ways of doing things.
Also feel free to ask any questions either as a reply or send me a PM. All four of my hermit crabs are PPs (purple pincers) for anyone curious.