The number of fax devices that can be supported varies based upon the operating system that the computer that hosts the device is using.
Windows Server 2003 Standard Edition, and its Small Business Server derivative, support 4 fax devices. Windows Server 2003 Web Server Edition supports 1 fax device. Windows Server 2003 Enterprise Edition and Windows Server 2003 Data Center Edition have no limit.
Windows 2000, both Professional and all server editions, support 2 fax devices.
Windows XP Professional Edition and Windows XP Home Edition support 1 fax device.