The leader starts out somewhat faster than the posted speed for the ride. Of the 12 starters, two assume this IS the ride pace and decide to go off by themselves.
The leader soon runs out of steam and slows the pace down too much. Two riders decide to ride ahead and miss a turn on the route.
Gratuitous Hills (hills for the sake of climbing hill after hill — or designing the route to include as many hills as possible) — leave the two sane riders with the attitude of why? They go off on their own.
Finally its pit stop, lunch, snack time. Two riders decide they need to hurry back and leave the group.
Back at the lot the leader says “Where are … and names all the people who dropped off”
So answer to the question in the title is: the number depends on the color of the leader’s helmet.
Another normal, or not, group ride put to rest.