How do you calculate the occupant load in a hotel dining room?

Assuming it seats 50 or more:

Under NFPA Life Safety Code, you would divide the net square footage by 15 square feet per person to obtain the area load limit, then divide the egress width by 0.2 inches per person to obtain the exit load limit, then select the lower of the two limits.

Your local code may vary, and there are many other factors to consider under individual circumstances of each occupancy (type of construction, sprinklers, fire alarms, what floor it's on, whether there are multiple means of egress, etc).