As far as I can tell by looking through countless missions, there is only one mission that actually requires ambulances as such. That one mission is 'plane off the runway' 
It says you need a required 2 ambulances as a minimum... Most times I would send at least that, but other times I may just send a mass casualty and chief for example if that is what units I have spare.
Is there any particular reason why this is the only mission that specifies the required ambulance count? If not, could it be removed as I see no point of inconsistency between missions... 