We have 7 peak units. They're staffed in 12hr blocks via OT. There could be anywhere between zero to all seven on at any one time, dependent upon demand.
I feel like you're describing what I call a "power unit." We keep a reserve truck at each of our stations in case something happens to our primary unit or we need to split each medic into their own truck. We also run an extra power unit that roams around and provides additional coverage during times of high call volume, storms, or just when extra people are scheduled.