You have too many blobs. You need to get rid of the parking spot lines first. Try imopen() to get rid of the lines. Then do a hole fill with imfill(). Then use that with imreconstruct() to get back the original cars without the parking lines. Now you have just the cars but each car may be made up of several disconnected pieces that you have to join together into a single blob. Do a hole fill again, or call imclose() and do a hole file, and then call bwlabel(). I haven't tried it but I think that may be an improved approach, though it might take some tweaking to make it more robust to other images.
Another approach might be to use hough() or houghlines() to find lines, then divide up the image into an integer number of parking spaces - a grid essentially. Then take the sum of the pixels in the grid (parking space). If there are more 1 pixels than 0 pixels then the parking space is occupied.
