One needs to find the number under the car.
If you look at the numbers upside down, they read 86, __, 88, 89, 90, 91. The number that fits this pattern is, of course, 87.
Therefore, the space as the picture is currently viewed should be filled in with L8.
I believe the most obvious answer is:
There is a usual sequence of numbers, we're just looking at it in the wrong direction. The numbers of the parking spots must be viewed from the outside in, as a driver would see them, which is upside-down from the current view.
The shown numbers are: 86, ??, 88, 89, 90, 91. Therefore the answer is 87.
The Number is 87. Just rotate the image 180 degree.