Hello, Walter Shrimp is a countable noun and has a regular plural: 1 shrimp, 2 shrimps. At a restaurant, you can order one shrimp cocktail or two shrimp cocktails. It's like potato salad or apple pie.
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Therefore: "This packet contains 25 peeled and salted shrimp."I'd be inclined to disagree. I would use 'shrimps' in this context.