Let’s see; you want Numlock to be on (enabled) when the system starts. After all, if your password has numbers it’s annoying to find out you can’t login because Numlock is not turned on after booting up and entering your password using the Numeric keypad.
Linux Mint provides a way to enable Numlock at login but the option is greyed out by default in Cinnamon, MATE and Xfce Desktop flavors, and an additional utility must be installed to make it functional; numlockx, a utility to enable the keyboard’s Numeric Lock during X11 session initialization.
Enabling Numlock at login
1 – Install numlockx from Terminal
sudo apt install numlockx
2 – Once numlockx has been installed, you can then proceed to enable Numlock at login:
Linux Mint Cinnamon: System Settings => Login Window => Options: Enable Numlock
Linux Mint MATE: Control Center => Login Window => Options: Enable Numlock
Linux Mint Xfce: All Settings => Login Window => Options: Enable Numlock
Numlock will be (and remain) enabled during subsequent restarts.