What does "alarm" mean, as in "2-alarm fire"?

‘A two alarm fire is a fire that is too much for one firehouse’s resources to handle. When multiple agencies or groups are called in, the alarm count increases. Two firehouses are called in, vs. three, four or five.

@gailcalled‘s has it right. So, when you hear two alarm or three alarm you know it was a big fire and couldn’t be handled by just the local firehouse.

Hence, 2,3, or 4 alarm chili from less to more heat in the spices. The bigger or more difficult the fire, the more alarms.The more, burn in the mouth, the more alarms.

So, as I understand it, then, it refers to the number of fire stations involved, as opposed to the number of calls received?

One, two, three, four or five firehouses are called to attend the fire.