It's an unpopular view, but except for RAInf, we've been an Army of two's for a while. And in most of our key capabilities we are an Army on ones.
It doesn't matter. The indivisible number for armour is 2, although the realistic indivisible number is 4. That's a Troop.
A Tp of tanks matches well with a Coy of infantry making a Combat Team. We fight as a combined arms force, which means our building blocks are CTs, not Coy/Sqn. If anything, an 18 Tk Sqn means that there is a left-over Tp when the Regt is matched with a Bn, allowing a heavy CT to be formed, or another Coy to be brought into the Bde providing more CTs, and hence provide more flexibility.
The only specific problem with 12 tanks is that you have no flexibility with the Sqn HQ - they have to fight in a CT (3x Tp of 4 tk versus 4x Tp of 4 + SHQ). That reduces the C2 flexibility available to the CO or Bde Comd.
Remember, a barracks structure is irrelevant to a war fighting structure. As long as there is an even number of tank, it'll be fine.