Work "up" on the blue-legged hermits. I think 1 per gallon can be way too excessive, especially if you have to $$maintain$$ that number because you think you "need" to. Great advice about the size of the tank, too! I like to think in terms of surface area and detrituts available for them to "maintain." Such being said, having 4-5 in a 10G has been totally sufficient for us in the past. But if we applied that "ratio" to say, our 100g reef then all of a sudden I have 40-50 in there. Not necessarily a bad thing, if you have plentyof rock surface... But when you start having a coral dominated tank you can see crabs starting to "pile up" real easily.

What to do? In a 55 you might start with say, 20 blue-legged hermits (if those are your crabs of choice - I like them, myself). Remember that crabs need to eat to stay alive, and they are pretty hearty scavengers. I truly don't think most folks have enough bioload in their 55g tank to support 55 blue-legged hermits. When you start needing to "supplement" their diet, it is a good indication that you've more than enough to handle the day-to-day detritus of a normally-stocked 55g. You can easily tell if your crabs are really hungry because when you drop in some Wardley's shrimp pellets they'll come scampering out like crazy even with the lights on. Remember, they are supposed to be a "nocturnal" feeder.
Now, some tanks have higher degress of "crab grazing" in them, and there's always
something of "major importance" for them to be devouring. But many systems that run with not that much food input, don't really
need a whole boatload of crabs.
Best to judge the "necessity" of a certain number of crabs based on the food input into the tank - not necessarily how many gallons the tank is.
Cheers