I'd say three could knock you out.

Poke the eyes, kick the balls, chop to back of neck.

Show me the man that isn't down after that, and I'll show you someone that's not a man ^_^

On a serious note however, what is the difference between a vital point and a pressure point? Most of the time, if I'm aiming for a target, it would be like the side of the elbow, middle of the bicep, middle of the thigh, the points that make that limb hard to use for a while. Which ones are these?

sort of going along the lines of what pess said the best pressure points i've ever used in contact are the solar plexus and the throat.

a good hit to the solar plexus (the area under the ribs where they come to an upside down v shape) interferes with the diaphagram and results in instant severe winding and if done well enough can force you're opponent to vomit.

a punch to the throat heavily restricts breathing thus stops the opponent fighting. the throat is a harder target because its obviously easier to protect.

either way they're not getting up and its a bit simpler than learning complex slapping/finger movements.

oh and the person who says the chop to the base of the neck doesn't work is wrong, i've seen it done by my old karate sensei to one of the more senior and willing students.

oh right. well whatever works lol