Zigbee limits are based on the amount of memory, but you shouldn’t run into them before you hit the general smartthings limit of 200 devices per location. And that includes all type of devices, Zigbee, zwave, LAN, cloud to cloud, and virtual.
200 Device Limitation? Really? Any alternatives?
There’s a separate limit on the number of Zigbee children that a hub can have connected to it directly without going through another Zigbee repeater. That’s 64 on most hub models.
FAQ: 32 Zigbee direct connection device limit?
But to be honest, the offline problem is one that affects many people with many different types of devices and just seems to be a legacy of the groovy cloud platform. Hopefully the new edge architecture will work better in this regard.