A 24x36mm "full frame" sensor is 1.6 times larger in linear dimension than a 22.3x14.9mm APS-C sensor. In terms of sensor area, the full frame has 1.6 * 1.6 = 2.56 times the area of the APS-C sensor.


Since pixel count is based on the area of a sensor and not its linear width/height, the appropriate math is


21 MP / 2.56 = 8.2 MP,


so Daniel quoted the correct value.