One needs to find the number under the car.
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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.