I am trying to control the rotation of a SG90 servo with a 10k Pot. The serial monitor shows that the pot goes from 0 to 1023 and I am just using the example code in the Arduino IDE called 'Knob', however the servo only moves between 0 and approx 850 and another after that the servo doesn't move.
That's fair enough. Servos actually work on pulse lengths and the full range from 0 - 180 in servo.write maps to 0.544 to 2.4ms. Some servos don't handle pulse lengths over about 2ms,which maps roughly to 850. You'll possibly find it doesn't move much between 0 and about 100 either.
Provided you're getting the full angular movement of the servo you're fine. That should be around 180 degrees for a real SG90 but there are many cloned SG90s about with all sorts of different specifications.