Can Plugzio Mesh work as a stand-alone unit? what is the limit?

Nope. Plugzio Mesh has to be combined with either one Cellular or Wifi unit to operate.

In technical terms, the gateway is Plugzio Cellular or Wifi and you can add several nodes (i.e. Plugzio Mesh) to a single gateway.

The limit depends on the signal strength of the gateway (Plugzio WiFi or Plugzio Cellular). At an ideal scenario a single gateway can support over 20 mesh devices.