What is a Cessna 172's maximum altitude?

So many variables.

How powerful is the engine today? How good is the fuel today? How humid is the air today? What's the temperature and air-pressure today? How turbulent is the air? How smooth are the lifting surfaces? Are they free from lumps and bumps? Are they polished smooth? How accurately has this wing been manufactured to correct aerofoil shape? How heavy is the total weight? How good is the pilot?

And those variables are ones just off the top of my head.