We have been struggling with the number of vibration motors we should put in our device, and we have decided that the most logical choice was to put 50 in it. Indeed, it would give us a 90 mm scale interval, which is pretty accurate, and we could even improve this accuracy by, for instance, setting 2 levels of vibration, which would give us a 45 mm scale level.
So the user will be able to detect the difference of distance between 2 objects that are 9 cm far (max).