At a selfie photo booth in an amusement park, customers form a line to wait for their turn to get a selfie. Only one customer is allowed in the booth at the same time. On average, a customer spends 4 minutes in the booth to take a selfie, with a standard deviation of the time estimated as 2 minutes. It is observed that customers arrive according to a Poisson process with an arrival rate of 12 customers per hour.

What is the average number of customers inside the photo booth?