This *6 is all" may come from something out of Lyman's 49th, page 50. They say that for typical rimless cartridges, a good case life is about 8 firings. For max loads, it's more like 5. Belted magnums might get two or three, with rimmed cartridges (e. g. 7.62x54R, .30-30) seeing somewhere in between belted and rimless.
My guess is that someone came up with the number "6" as an attempt to somehow "average" all these possibilities.
.38 Special cases will very often last well over 20 firings (yes, I've done this, and those cases are still goin' strong). Remember that the .38 Spl round is a pretty low-pressure round. Same for .45 ACP. Now, .357 Magnum loads will reduce that case life.