No, there is no such limit as long as all the devices paired are Wi-Fi enabled.
To begin with, there can only be 1 administrator to prevent access crowding. However, this admin can go on and provide access to as many individuals as he likes, all of whom will receive notifications from the Sure Bell.
No, the Sure Bell is not compatible with these devices.
Windows devices would need Windows 10. Android will work with any version 4.0 and higher while Apple will require iOS8 and after.
As long as your device is connected to the Wi-Fi network, there should be no problem.