How many times do you have to sex to have a baby?

Answer 1

One.

This does not mean "If you have sex once you are guaranteed to have a baby", it means "If you have sex once there is a non-zero possibility that you will have a baby."

To put it another way, it's like if you asked "how many times do you have to roll a pair of dice to get a 7?" On any given roll there's a one in six chance of it being a seven, but it could happen on the first roll, or you could roll 100 times or more before a seven comes up (this is not very likely, but it is possible).