I notice flow of traffic in and out of the local convenienstore is quite regular but what if 500 people all decided to go at the same time?

No.

Companies do traffic studies to determine personnel requirements.  

I don’t know about everywhere, but 500 people would have a hard time parking at the convenience stores I’m familiar with. 
I think that after a certain number, the rest would conclude the place was too crowded and go elsewhere. 

With Demographic, Psychographic mapping along with traffic studies nothing is left to chance. So theoretically it is possible however not very likely.  What you are observing is good planning based on observation - nothing magical at all

I predict there would bee a line outside the door---if everyone decided to wait.

Cause people just go on to the next station if one looks too busy.