How many editions (not to mention the 5e playtest) has there been a version of Keep on the Borderlands for?
Probably fewer than you think.
It first come out in 1980.
It then shipped with the Moldvay set (1981-2). It was
not in the Red Box (Mentzer) Basic set of 1983.
It went out of print in the mid-1980s.
The 10th Anniversary D&D Collector's Set in 1984 had it - 1000 copies of that was made.
The Caves of Chaos portion was reproduced in B1-9 In Search of Adventure (1987).
It was reprinted in full for the Silver Anniversary Collector's Set (1999).
The Caves of Chaos portion was used during the playtest. (2012).
It was reproduced in full in the prestige product, Into the Borderlands (Goodman Games, 2018).
Modules inspired by it:
- Return to the Keep on the Borderlands (AD&D 2E, 1999).
- Keep on the Borderlands: A Season of Serpents (D&D 4E, 2010)
Actual editions that printed a version that was the original updated to the new stats? Basic D&D (Holmes), Basic D&D (Moldvay). D&D Next playtest.
Return to the Keep on the Borderlands has the same basic plan, but greatly changes the cave descriptions (and Keep descriptions). I wouldn't count it as a reprint. Likewise for A Season of Serpents.
Cheers,
Merric