Dragon88 is basically correct.
However, depending on the vintage, they may have been intended for 6 in the gun and 6 each in two dump pouches. When the speed strips came out they were designed to fit into the dump pouches. So maybe 6 in the gun and 6 in each of two speed strips. I don't know who originally came up with the idea of only 12 reloads, but it stuck. Probably the FBI, in they're all knowing wisdom!

My department never bought/used ammo sold like that. We always used reloads which always came in 50rd boxes. Obviously there were departments, with much fatter budgets, that could afford factory manufactured ammo in 18 round boxes. As a matter of fact it probably was a govt agency like the FBI. They always buy factory ammo for training. At any rate they're very collectible, at least to those of us who collect such things!