Turn NumLock on on startup?

There is a package in community numlockx:

# pacman -S numlockx

and then add it to your .xinitrc:

numlockx &

There are also methods on the the Arch Wiki if you are using a login manager, such as GDM or KDM: https://wiki.archlinux.org/index.php/Activating_Numlock_on_Bootup

The archlinux wiki has a page about enabling the numlock key on bootup here

Numlock status at startup is usually an option in the BIOS (as another option, rather than setting it per OS). Although I guess it's possible that it's already set to on, and something in Arch Linux (or other OS's) is turning it off again.