Elmer T. Lee is usually credited with the concept of "Single Barrel", as a way to bring Bourbon to the attention of people who were enamoured to Single Malt Scotch. There being no bourbon equivalent (or it might be interpreted as universal equivalent) to single malt, Elmer's idea was to market output from a single barrel as if that were something special. Customers overwhelmingly thought it was, and partially that's because the claim was accurate in the case of Blanton's.
I'd be inclined to believe the term "small batch" was dreamed up (uh... sorry; I meant to say
"was the brilliant, creative marketing plan driving the decision") in order to catch up with what Ancient Age (or whatever they called that distillery) was doing.
That was in 1984. At around the same time, Booker Noe was gifting personal friends (and possibly big customers & corporate mucky-mucks) with hand labeled bottles of uncut, unfiltered, ~eight-year-old bourbon. Someone at Jim Beam Brands, probably the same who decided to present Booker to the public as a sort of Jim Beam's answer to Colonel Sanders (Kentucky Steamed Corn?), built a four (I believe originally five, but that's another story) brand "set". These were not "single barrel", and Beam had no intention of going that route (to their credit, in my opinion). So they came up with the "small batch" concept to give the impression that these were carefully crafted bourbons, made in very small quantities. Rather than the truth, which is that they are dumped from carefully SELECTED barrels of regularly-produced whiskey, BOTTLED in small quantities.
At least I THINK they came up with it. I've always thought it was fascinating that Kentucky Bourbon Distillers ALSO market a collection of four individually-branded special bourbons (Noah's Mill, Rowan's Creek, Kentucky Vintage, and Pure Kentucky XO),and THAT collection is called "Small Batch". I don't when that came out, although it probably was sold in Japan for awhile before it was released here, and nobody seems to talk about it, but it sure seems as though someone stepped on someone's toes there.
Anyway, today there is only one bourbon distillery that DOESN'T feature either "small batch" or "single barrel" offerings. So it shouldn't surprise anyone that ADI's source for it's "19 barrels or less" was Maker's Mark, the only distillery that doesn't market such a product.
To the best of my knowledge, the only truly SMALL BATCH bourbon offered to the public were the three experimentals from Buffalo Trace and Woodford Reserve's four-grain. The former three were BARRELED as one barrel apiece, about as small as one can get. The latter was MADE as a small batch, or at least as close as we get today; the pot stills at W.R. operate somewhat more like continuous distilling than true batch distilling. All of the other "small batches" are only accurate insofar as they try not to bottle more of it than they think will sell quickly (as opposed to "commodity" whiskey that can be warehoused as bottled product like ketchup or beans or any other packaged product).