Good question.

Let me try: suppose there are 200m cars in US, on average each car goes to gas stations once a week, then about 30 million cars are served everyday. Let us assume each gas station serves a car every 2 min. on average over 24 hours, then it can serve 24*60/2=720 cars every day. So roughly there are 30*1000,1000/720=40,000 gas stations.