At the vollie squad, we've got a main cylinder (M, I think), a D cylinder in our medical bag, a D cylinder in our trauma bag, and 1-2 spare O2 cylinders, usually with regulators.
At the transport co, there is a main cylinder, a D cylinder in a sleeve, and 1-2 spares. The ALS trucks have the D-cylinder mounted to the stretcher and have 2-3 spare O2's. When we work events, we have a pair of event bags, 1 of which has a full set of oxygen supplies including a D-cylinder. This either supplements what is on the truck, or acts as our only O2, depending on the event.
At the new paid job, we've got a main cylinder (M, I think), a D cylinder in a sleeve, and a D cylinder in the CPAP bag rigged up with the quick-connect for the CPAP. We also have 3-4 spare D cylinders. We also regularly swap out O2 with the PD, because we have regulators integrated into all of our tanks. If the PD's O2 is running low, we will take their tank and they get one of our "spares".