The calculated distance (air line) between Katwijk aan den Rijn and Amstelveen is approximately 20 miles respectively 32 kilometers.
By car or train, the actual journey to Amstelveen is certainly longer, as only the direct route (as the crow flies) between Katwijk aan den Rijn and Amstelveen has been calculated here.
Air line (approximately)
32 kilometers
17 nautical miles