Within the local response area - whatever is required - 1 or 2
Elsewhere within the group - whatever is requested - usually 1 only.
Anywhere else - 1 only. (unless the second truck also responds
because it has enough crew and the fire or whatever is nearly in
our area. In which case there will be 2.)
If this is not sufficiently confusing, I can reword it longer &
more convoluted
Oh, and if I get on any appliance, it will usually not be required.
This is why I am
stopcallking