How many people are wearing black hats?

A group of people have a dance party with a hat on their heads. There are only black and white hats, and at least one black hat. Everyone can see the colors of others' hats, but not their own. The host will show you what hats others wear on their heads and turn off the lights. If someone thinks they are wearing black hats, they will slap themselves in the face. Turn off the light for the first time and there is no sound. Then we turned on the light and watched it again. When the light was turned off, it was still silent. It was not until the third time that the light was turned off that there was a slap in the face. How many people are wearing black hats?

Let's guess, hey, hey...