A poke bowl shop is looking to provide sufficient staffing to meet the customer demand at a local marketplace. When customers arrive in the shop, they can fill out a menu form to customize their order. They submit the form to the employee, who processes the order, and collects payment. The arrival rate is 20 customers per hour. The service rate is 11 per hour. There is a lot of food options at the marketplace so customers will easily choose a different food option if they feel that they did not receive quick and superior customer service. The owners estimate that the cost of customer waiting is estimated at $25/hour per customer waiting and the cost of each counter server is $18 per hour, what is the optimal number of servers that should be working during the peak time?