Virtual Box can provide up to eight virtual monitors for a guest operating system. These will be presented in multiple windows on the host. Only in fullscreen mode (or seamless mode if supported by the guest) we can address the physical monitors attached to the host.
However, in full screen and seamless mode, they will use the available physical monitors attached to the host. As a result, for full screen and seamless modes to work with multiple monitors, you will need at least as many physical monitors as you have virtual monitors configured, or VirtualBox will report an error. You can configure the relationship between guest and host monitors using the view menu by pressing Host key + Home when you are in full screen or seamless mode.Virtual Box Manual