Replace the question mark with a number

A young woman asks her mother, "Mom, how many kind of penises are there?"
The mother, surprised, answers, "Well, daughter, a man goes through three phases. In a man's twenties, a man's penis is like an oak, mighty and hard. In his thirties and forties, it is like a birch, flexible but reliable. After his fifties, it is like a Christmas tree."
"A Christmas tree?" the young woman asks.
"Yes, dried up and the balls are there for decoration only."