Your NordVPN account supports up to 6 devices connected at the same time.
Connecting multiple devices to the same server
If more than one of your devices connects to the same VPN server simultaneously, each device needs to use a different VPN protocol to do so.
Using a router
Connecting your router to NordVPN counts as a single connection, but it protects every device on that network through it — a convenient way to cover devices that don't support a VPN app directly, like smart TVs or game consoles.
For more detail, see NordVPN's own FAQ page.