There are limits, but it’s unclear to me what they are. And they seem to be changing.
FAQ: Limits and Guardrails (max # of automations, devices, etc)
Tagging @jody.albritton
My own rule of thumb going back a long time, and not ST-specific, is never to put more than 30 zwave/zigbee home automation devices in a single group/scene/rule. I daisychain groups/zones if needed. (Easy to do in ST with virtual switches as proxies for a group.) Mesh networks just weren’t designed for massive traffic.