If its the same county to county, then it does mean there are only 2 ambulances available. You'll hear the same thing from AMR on the EMS Dispatch TG on the BOEC trunk. It gives the the ambulances that have completed their transports to hospitals a heads up to become available. It also means that ambulances will be moved into the area from other counties, ie; Clackamas County AMR covering Multnomah County AMR, usually the Sandy ambulance moving into east Gresham and a Clackamas ambulance moving to SE Portland. The same goes for Multnomah County ambulances moving into Clackamas County for coverage.
Well, I did notice that they only had seven ambulances responding to calls throughout the county from 10PM to 6AM, and then calls afterward were taken by different ambulances. At the time I heard this, two were just arriving to their respective calls, and one was being dispatched. So, if they already had one or two at the hospitals, this would make sense.
I also remember that the last ambulance to be dispatched before the notice was broadcast was responding from Metro West's HQ in Hillsboro.