A fire department in a large city certainly has a difficult job but its mission is fairly straightforward. When a fire is detected, the fire department dispatches an appropriately sized staff to assess, contain, and put out the fire, clean up, investigate what happened, and prepare themselves for the next blaze.
Yup, a pretty simple process when a manageable number of fires are burning but what would happen if there were hundreds or thousands of simultaneous infernos? My guess is that a senior fire chief (and perhaps other participants from local government and law enforcement) would have to make decisions on which blazes to resource and which to ignore. These decisions would certainly be based upon information analysis and best practices but there is still some risk that the disregarded fires would end up being far worse than expected, turn into disasters, and call into question the judgement of all involved.