When you say One WLAN Controller the only thing you can mean is a wireless lan adapter/controller card. This is also know as a wifi adapter. A WLan controller can scan multiple networks but it can only connect to one. So you can read in all the surrounding networks and their signals/security but you can only join one.
You need to be on a network to perform tasks on that network or its objects,
Karl, the answer is not correct, it is for a Wireless adaptor, but that is not a Wireless LAN controller, which is a completely differnet thing altogether.
A Wireless LAN controller is a device that controlls a number of wireless access points, and provides a common platform for their configuration. The access points are not as 'intellegent' as a 'normal' AP, and the WLAN Controller takes over this function. It allows a number of APs to work together in a more harmonious fashion. For instance, if one goes off, the controller realises and adapts the coverage (power) of the others to compensate. The WLAN controller usually also provides a common security policy, QOS and mobility. For more information on the Cisco solution, and about WLAN controllers in general, have a look at http://www.cisco.com/en/US/products/ps6366/
So in answer to the original question, yes they can perform the tasks for many access points. The number will depend on the manufacturer and the model, and sometiems the licence on the device. As for the networks, this is sometimes the case, but it depends on exactly what you mean by that, but I would guess the answer is also yes to that for most circumstances.