Technically, only one.
Peeta tells Caesar that he killed Brutus in the 75th Hunger Games. This was his only direct kill.
Many people say that he killed Foxface too, but he didn't. Foxface died because she stole some poisonous berries from Peeta. Peeta hadn't fed them to her, he didn't even know that she had taken them until she died from eating them.
And in the 74th Hunger Games, Peeta just pretends that he killed the girl who lit the fire next to Katniss's tree. This was a lie to build the Careers' trust, so he could protect Katniss from them. The only reason the cannon didn't fire when Cato killed her, was because the Gamemakers were signalling that Katniss was treed next to them. Peeta went back to 'finish her off' but didn't do anything since she was already dead.
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They are hoping that Peeta will lead them to Katniss so they can kill her.
That Peeta has teamed up with the Careers and is helping them find her and kill her.
because she will have to kill him
She doesn't get to see Peeta because Peeta wants to kill her. Peeta was kidnapped by the Capitol and they used trackerjacker venom to corrupt the images of Katniss in Peeta's head so that when he sees her, he will try to kill her.