Do I need to install and license any software to use instant support?

By design, there's nothing to install and license on the remote Windows or Mac computers of customers or colleagues ("end users") you want to support. All an end user has to do is run a small, disposable app on demand.

For end users on Android or iOS, they will need to install the VNC Server app from their device's App Store.

Each of your technicians providing instant support must download and install VNC Viewer on their desktop computers. However, they don't need to license the software. Providing you name these people as technicians in your RealVNC account online, licensing is handled automatically.