Why is the Verification Code asked on every login to Supercell ID?
To stop Supercell ID from asking you the verification code every time you want to log in, be sure to tick the box for "Remember me on this device".
If you are playing several accounts, you get the best experience when you attach all your game accounts to Supercell ID. This way you can switch between all of your accounts without entering verification codes.
The "Remember me on this device" tick box is under the field where you enter your email address.