Nobody likes waiting in lines. For many hotspots you have to wait to get in to the club, wait for drinks, and even wait for the bathroom. Sometimes lines are purposefully kept to build buzz around the venue, sometimes they are just a result of poor management.
There are different optimal rates for different services within a nightlcub. Depending on the venue, the outside lineup should be building a sufficient amount of buzz, while not deterring potential customers. Additionally, the lineup for waiting for drinks should be as short as possible. You can overstaff your bartenders to solve this problem, but then depending on how tips are split they may be unhappy about it.
Quick remedies are obvious, pushing bottle service and adding additional servers or bartenders can help, as well as differences in physical set up of the environment.
Interestingly enough, this can be broken down to a mathematical problem, within the realm of probability. Without getting too deep in to the numbers, we consider the nightclub to be a stable system. People are constantly waiting in line to purchase a drink, getting served, and moving elsewhere in the club. If there was one bartender servicing a club, the arrival rate would exceed the exit rate causing overflow eventually.
The best solution is to optimize this rate, so that customers wait the least amount of time while the minimal number of bartenders are servering. There are a few variables that affect these rates; (i) number of bartenders, (ii) number of guns, (iii) demand for drinks, (iv) average speed of mixing drinks, which varies on
Number of customers waiting in line for a drink = Arrival Rate X Time Waiting For a Drink
You could analyze these rates and determine exactly how many bartenders are needed. It may be easier for smaller clubs just to experiment with different numbers of bartenders, but for massive clubs holding thousands of people this might be the quickest way with minimal loss in profits.