Just touching the M&M's deforms them slightly. If you flatten the sides of the M&M's by having them rest on a sunlit table, you could probably stack six or seven.

Also, it is a candy coating, so if you stack a few then let them sit there for a few days (or minutes) in a room with the correct temperature and humidity, the candies will stick together.

Stack five pool balls then I will be impressed. Or even basketballs.