Replace the question mark with a number

An Amish boy and his father were visiting a mall. They were amazed by almost everything they saw, especially two shiny walls that could move apart, and back together again.

The boy asked his father, "What is this father?"

The father (having never seen an elevator) responded, "Son, I have never seen anything like this in my life, I don't know what it is."

While the boy and his father were watching wide-eyed, an old lady, limping slightly, and with a cane, slowly walks up to the moving walls, and presses a button. The walls opened, and the lady walks between them, into a small room. The walls closed.

The boy and his father watched as small circles of lights with numbers above the wall light up. They continued to watch the circles light up, in reverse direction now. The walls opened up again, and a beautiful young blonde stepped out...

The father said to his son, "GO GET YOUR MOTHER!!!"