How many people will five pounds of fried fish feed?

It depends on the type of fish:

25 pounds for white fish

50 pounds for shellfish without the shell

100 pounds for shellfish with the shell

20 pounds for oily fish (like bluefish)

40 pounds for red fish (like salmon)

and on the preparation: for instance if you're making sushi you'll need only about 4 pounds, if you're making sashimi or ceviche you may need 50 lbs.