Depends on the device type.
Portable devices like IR_BLASTER and SMART Travel Adapter can save upto 4 Wi-Fi Passwords to connect on availability and signal strength.

To add more than one wifi in those devices, Go to Settings>Devices>(Select Device)>Configure Device. Just add your new wifi in configuration screen

Whereas SMART Switch, SMART 16A Plug can save only 1 Wi-Fi Password.