For the 802.15.4 product, there is no limitation implemented with in the Code for the total number of end devices. So in this case it is a function of how many 16 bit addresses that can be assigned out.
As for the Zigbee product, depending on the specific version of the hardware and firmware you are using, a parent node such as a Coordinator or router can each support up to 10 - 12 End device children.