Question:
That said, in the USA, fire departments classify fires by the amount of equipment and firefighters that are needed to control it. A "one-alarm" fire might need a single truck, a "three-alarm" fire is quite a bit larger. From what I have heard, the biggest fires could be three, four, or even five "alarms" depending on the city.

Answer: You're right. "Alarms" have to do with the amount of personnel and equipment needed to extinguish the fire. What I've heard, though, is that the "alarms" are counted from an original and subsequent assessments of the fire. If enough manpower and equipment are sent in the first place, it remains a one alarm fire. If the officer in charge feels that more equiment is needed, it becomes a two-alarm fire. If still more equipment is needed, it's a three-alarm fire and so on. Relying on assessments to qualify a fire does not sound very scientific to me. I mean, the same fire could be a one-alarm if the original assessment was correct or a two- or three-alarm if it was misjudged at the beginning. But, like I've said, that's what I've heard.