ObjectDetections gives the output as a structure corresponding to the number of generated detection as a cell array of objectDetection objects. The given is generated as a result of drivingRadarDataGenerator function which gets added whenever you add a sensor to your ego vehicle in the driving scenario generator. As you've interpreted correctly, 6*1 cell would mean that 6 objects were detected.
The same can be visualised in the following example scenario I created:
On exporting the driving scenario, and running the function it would give an output of ObjectDetection with a 3*1 cell array