I hate numlock. I have 'enable numlock on boot' disabled in BIOS. When I boot to windows, numlock is not on. When I boot into Linux and sit at the login screen, numlock is not on. When I enter my password and enter i3, numlock is on. Case-insensitive grep for 'num' in .i3/config is empty. What's going on?
Since you did not specify much details about your system there's not much to go on.
I would recommend to download "numlockx", since I don't know which distro you are using I can't tell you how, but you should know how to install applications...
When numlockx is installed, just type this into your i3config:
exec_always --no-startup-id numlockx off
I hope that solved your issue.