You can use SimpleLink on unlimited devices within one household. The number of allowed devices refers to the number of different IPs allowed to connect to the VPN at the same time (simultaneously).
Usually a mobile device when connected to 4G/5G will have one independent IP. In a household Wi-Fi environment, all devices connected to the router share the same IP, which is the IP of the router.
If the plan you purchased has 3 allowed IPs, then only ONE router can be connected at the same time (1 IP occupied). However, you can also connect a mobile phone and a computer when you are outside home (another 2 IP occupied). Three Allowed Devices (IPs) are enough for 99% of users.
If the number of devices/IP exceeds the allowed number of the Plan, the system will prevent the new device from using the VPN traffic (You can login and connect but no data will flow.) and report it to the administrator.
Frequent violation of this rule may trigger risk control. If you need more allowed devices and IP, please contact customer support.