Clean up crew suggestions for 120 gal

[Oxymoron]

I was browsing through a website and read...

• Question: "How many snails and crabs do I need for my tank?"
  Answer: A good rule of thumb is 1 crab for every gallon, and 1 snail for every gallon and a half. This is a good starting point, from here you can judge for yourself whether or not your clean up crew needs some additional recruitments.

I'm a little bit reluctant to believe that you would need so many critters?