A total of 6 devices can be connected with 1 NordVPN account at the same time. However, there is only one limit. If you connect devices to the same server, you must choose different protocols for the VPN connections (TCP and UDP are different protocols hence you can connect one device to TCP other to UDP on the same server). That means that a total of 4 devices can be connected to a server at once - through L2TP, PPTP, OpenVPN TCP and OpenVPN UDP. If you have two more devices at home you can connect them to another server with any two protocols.
However, if you have even more devices at home, you can set up your router with our VPN. The router would use only one device slot but all your devices connected to the network will be under VPN protection.