How many licenses do I need
Radmin 3.x consists of Radmin Server and Radmin Viewer parts. If you want to control computer A from computer B, you should install Radmin Server at computer A (for example your office PC) and Radmin Viewer at computer B (for example your home PC or notebook).
Radmin Servers should be licensed (see more about purchasing a license). Radmin Viewer is free, so you are able to install it on any PC you are willing to have the remote control from.