Witch and dwarfs

One day, a witch arrived at a dwarf village, where 10 dwarfs of different heights live. Instead of killing the dwarfs instantly, the witch decided to play a game with them. She said she would put a hat, which could be black or white, onto the head of every one of the dwarfs. Those who correctly guess the color of their own hat would be spared. What's the optimal strategy to minimize the number of dwarf deaths? Using this strategy, how many dwarfs risk death?