Do I need to install and license any software to use On-Demand Assist?

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.

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 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 the online management portal, licensing is handled automatically when the technician signs in to VNC Viewer with their RealVNC account.