How much water must be added to make 500 ml of 12.0 M HCl to 1.0 M HCl

(500mL• 12M) / 1.0M
=6000m

Is this correct?

What's 6000m? Did you mean 6000 mL? No, not quite. That's what the total volume must be; assuming volumes are additive you would add 6000-500 = ? mL.