If your Windows anti-virus license says that it is a single user license, it means that it can be installed only on a single Windows computer at a time. But some anti-virus that are total security options may allow you to install it on all of your devices. This means that they can be supported on macOS, Android, and iOS, along with Windows. In other words, while such anti-virus license will only have a single user, they can have multiple devices of different platforms and operating systems.