Found the following documentation from ITEAD product list. This is consistent with the behaviour I have seen. Without a Zigbee bridge added, its only 10 end devices.
NOTE: ZB Bridge-P can connect 26 sub-devices directly (10 end devices + 16 routers, or 26 routers). If you want to connect to 128 sub-devices, routers are required, and one router-device can expand to connect 20-32 sub-devices.