The approximate difference between Earth’s equatorial radius and polar radius is 21 km.
Earth is not a perfect sphere: its rotation makes it an oblate spheroid, slightly wider around the equator and flattened at the poles. The WGS84 reference ellipsoid gives an equatorial radius of about 6378.137 km and a polar radius of about 6356.752 km. Their difference is approximately 21.385 km, conventionally rounded to 21 km.
This difference is between radii, not diameters. The corresponding difference between Earth’s equatorial and polar diameters is about 42.8 km. Confusing those two measurements is why answers such as 50 km can seem plausible.
The effect has practical consequences. Latitude and longitude systems, GPS, satellite navigation, and geodesy use an ellipsoidal Earth model rather than a simple sphere. Earth’s average radius is often quoted as about 6371 km, but that is a mean value and should not be substituted for either the equatorial or polar radius when comparing the planet’s shape.