If everything from the cut to the control box is indoors, then you don't need any specialty wire. Any 18 guage wire should do. The principal thing about "irrigation" wire is that it has an outer sheath that allows the wire to be buried directly in the ground. The sheath protects the wire from degrading because it's in contact with the soil. But once you get indoors, you no longer have that sort of worry.
Oh, and by the way, when you have 6 control valve, usually you only have 7 conductors: 1 common, 6 control lines. You should only need 12 conductors if the valves are spread around the yard (and therefore two wire MUST go to each valve). But if the valves are located at a single manifold... well if someone wired it with 12 conductors, they were wasting $$$ buying more wire than was needed.