The 15P has 6 on-board zones and supports up to 16 expansion zones. "Expansion zones" means either hardwired (4219) or wireless (5881) expanders, or both. So yes, it can support two 4219 HW expanders.
So a total of 22 zones, _including_ the on-board hardwire zones.
Magnum Alert, you may have been thinking of the old 20SE, I seem to recall that supported 32 zones total.