Depends on the LEDs. Newer generation LEDs have lower forward voltages, so they can drive more LEDs.
New driver boxes will be coming for the larger drivers. We can always get something custom made ahead of time, but waiting for us to finish production versions will save you a lot of cash. Giving the shop a call never hurts.
Yup. And everything is driven in series. You do have to be more cautious around the higher voltages, but the same can be said for any driver. My last high power fixture that I built was using 4 Bridgelux 1203 neutral whites (18W each), and 32 XR-E royals, driven off just two 120W drivers. That's almost 200W of LEDs.