I'm pretty sure no one could answer this since so many die in buildings and off-panel throughout the years. If you count the Emperor Joker run, it could be in the millions

I heard it was around 3000.

Its in the quad digits.

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It's over 9,000

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Joker once said that he lost track a long time ago but loves that Batman knows the number.

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It's over 9,000

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It's like asking an exterminator how many insects they've killed...

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It's over 9,000

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I think he gives a specific number at one point in The Dark Knight Returns when he's on the TV show, but I can't remember what it was exactly.

If we count EJ he ate China and destroyed the world... That's 1-7 billion

Thousands of people. If you wanna count Emperor Joker, billions of people were slaughtered

Joker's killed at least 2.000 characters, I think, but the major ones were Jason Todd and his mother and Commissioner Gordon's wife.

Who really cares, though? The guy's insane and a great strategist. Those two things don't go together but earlier comics reveal that he is indeed crazy or "super (in)sane," so he's not responsible for any of it. It might be a little like giving a little baby access to a gun; the problem is we're dealing with someone maybe more intelligent than Stewie Griffin, so it's pretty hard to keep the weapons away. I also want to believe that he, like Deadpool, knows he's in a comic, so he knows he's not doing anything wrong and gets a good laugh out of the people who think it's all real.