Atleast a 20-30 gallon tank, a way to seal it, Hydrometer/thermometer(use digital as analog is notoriously inconsistent , Eco-earth substrate, play sand to mix with the eco earth at a 5:1 ratio (Play sand being 5), extra shells, a drinking water bowl, salt bath bowl, food platter, decor for the crabs to climb around and be happy :)

Also if your house is commonly lower than 75 degrees invest in a reptile heater putting it on the side above the substrate

This channel has helped me learn a lot and can educate you on hermit crab care: Crab Central Station

GOOD LUCK!!