It really depends on what you use them for at what you work on. I have about twelve, but I use hammers a lot, and could use a few more types and sizes. 8, 12, 16, 24, and 32oz ball peens will help with a wide variety of projects, but if you are banging panels, you're going to want some body hammers. Everyone needs a ~16oz finish claw hammer, and if you ever want to frame anything, a ~24oz framer. At least one good sized dead blow and a ~32oz drilling hammer for stubborn objects, an 8lb sledge for yard work... How about a non marking mallet? A brass hammer? A chipping hammer? Hammers for masonry?
Not every style do you need multiple sizes, but for some, you really do (such as ball peens). I definitely see some ridiculous redundancy on here. I don't understand it, but if you're looking to do a significant amount of pro level work in a given field, you're going to want a good complement of hammers for that, and the know how to use them. Hammers are probably the most misunderstood tool.
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