How do you calculate the load occupancy of a bar?

Net floorspace divided by load factor from occupancy tables of applicable code. For instance, NFPA 101 requires 7 sq ft per person in compact dance areas and 15 sq ft per person where there is seating, provided it is not fixed seating.

You then calculate the egress load by allowing one person for each 0.2 inches of doorway or hallway (whichever is narrower), or 0.3 inches if there are stairs.

The lower number (area load or egress load) is the overall limit for that occupancy. There are, however, many other factors for each specific location, such as whether the main doors can accommodate at least 2/3 of the max load, whether there are sprinklers, whether there is live entertainment, what floor of the building it's on, what kind of building structure it is, etc.