I have a surprising number of hammers, and I use every one of them.
If you're going to work on ANY sheetmetal, you need a dedicated autobody hammer. The large-diameter head keeps the surface of the metal nice after dining out little dents.
And of course, the more times you use it, the more you'll be comfortable working sheetmetal, which means the more specialized body hammers you'll need. I think I have 7 or 8 right now, and there are two or three more I'd like to get. That should do it for me.
Then I have five different sizes of ball-peen hammers, from 8oz to 2 pound. Different hammers for different chores... bending flat-stock in the vise, working with different sized cold chisels, generally beating on stuff.
When the biggest of them doesn't work, I have a 3 and a 5-pound sledge (drilling hammer). Those work well for driving spikes in yard projects too.
When the smallest of them doesn't work, I have a very tiny ball-peen, which is actually the most used tool I have. It's just enough wen you need to apply more force than your finger tips can deliver. Or more. The last time I used it was to separate the transmission from the back of the block in my '01 Dodge Dakota... it was just enough persuasion with a small chisel acting as a wedge. My buddy was shocked.
I found one just like it on GJ, and talked him into buying it, and now he uses it all the time too.
I've got a mallet with a plastic head on one side, and rubber on the other that I just used tonight. (Craftsman) and use it a hell of a lot.
I have a rubber mallet that I don't use all that often... couple times a year maybe, but when you need it, it's perfect.
Don't have a lead hammer yet, but I have the mold. Looking for the right handle, and a hand grip.
And of course, I have a basic fiberglass-handled claw hammer (Craftsman), and an ancient wood-handled claw hammer (also Craftsman).
All told, I have about 20 hammers (counting the one in my road trip box, and the one in the wife's Corvair trunk box).
And darn if I don't use them all.
-Brad