How 18 Sea Dart ships translated into six available

The planned Sea Dart force comprised 14 Type 42 destroyers, three Invincible-class carriers, and HMS Bristol, the sole Type 82 destroyer. That made 18 fitted ships, but the stated guarantee was only six available at once. The speech does not explain the status of the other 12 or how that figure was calculated. Even so, the three-to-one gap shows why fitted hulls and immediately available ships are different measures of fleet strength.