Depends on the model but it's up to 16,000 sessions for the largest ASR based CUBE, that's a few years old as I haven't had anything to do with CUCM/CUBE for a few years now.
The limiting factor (beyond licencing) is the hardware or hypervisor hardware.
Biglebowskis Razor - with all things being equal if you still can't find the answer have a shave and go down the pub.