You may need 2 to 4 as they are motion activated. 4 will definately create an invisible barrier round the car but you may be able to get away with 2 or 3 depending on how good the sensors are. If you park up against a garage door and you have the room then 1 on each side of the car at a wide angle so that the sensor range triangle over laps beyound the boot.
If you know the cat owner and it is causing real damage to the car it is very cheap to bring a small claims court action at that level.
Find a wax or spray that is citrus or pine they hate that smell and won't go near it.
Try a couple of lemons or oranges cut in half in the scuttle or by the area it jumps up.
At least you can say the cat has taste
If you know the cat owner and it is causing real damage to the car it is very cheap to bring a small claims court action at that level.
Buy an outside cover like mine has, it might prevent cats from making marks in the paintwork but it won't stop kids from stoving the roof in