how many power strokes an i.c. engine performs?

Answer 1

It depends on if it is a two-cycle or a four-cycle internal combustion engine.

Two-cycle motors deliver one power stroke for each revolution of the crankshaft.

A four-cycle engine requires four strokes of the piston (two up and two down) and two revolutions of the crankshaft to complete one combustion cycle and provide one power stroke. 1 is the air intake, 2 is the compression, 3 is the power stroke, and 4 is the exhaust.

Answer 2

Each cylinder only performs one power stroke per revolution.
On a 4 stroke engine:
http://auto.howstuffworks.com/engine.htm
1- the piston moves down and draws the fuel air mix into the cylinder and the intake valve shuts
2- the piston moves upward to compress the mix and the spark plug ignites
3-the explosion pushes the piston downwards (power stroke)
4. the piston moves upward, the exhaust valve opens and out goes the burnt mix... start all over again.

And on a 2 stroke:
http://www.howstuffworks.com/two-stroke2.htm
1-piston move upward pressing out the exhaust and compressing the fuel air mix, sparkplug fires
2- piston is forced down (power stroke) start all over again.