One purchase gives you the right to use Sighthound Video on one computer. The Pro Edition license is for an unlimited number of cameras on a single computer, so if you want to run more cameras than your computer can support, you will need to buy one license per computer. We have found that inexpensive quad core computers work well with 8 cameras and can be purchased for $500 USD, so you could deploy just two computers for 12 cameras and have room to expand. See System Requirements
If you have Sighthound Video installed on multiple computers, you can only view video clips on the computers that were used to record the clips. You can however use the software that came with your network camera, and access the web page provided by the camera manufacturer, at the same time you are using the Sighthound Video software.