How many Flettner®s can you see?

Can you count the Flettner® Ventilators in the image below?

Complete Train

Is the answer a) 24 , b) 18 , c) 20 or d) 22?

There is clue in the text below.

Phil Kent of Alan Keef Ltd, the company who fitted the Flettner® TCX-2  ventilators to the passenger carriages of this leisure railway states, “The ventilators work very well at low speed, we run at around 8 to 10 mph max. They produce a noticeable air flow in through low level louvers and out thru the roof ventilators.

The carriages are totally enclosed as they pass through animal enclosures so it is essential to have plenty of air flow which is why we fitted 4 per carriage”.