This is always an interesting topic, and it has been discussed in other threads, when rarity comes into question. The likely reality is, no one can ever know, for sure, how many copies of a certain poster exist. Sure, there are copies that will appear for auction, or sale, but, as T said, there are also collectors who own copies who have no intention of selling anytime soon.
So if a certain style poster (whether it be
Barbarella or any another) is popping up a couple times a year for auction, it would appear to be very unlikely that only 20 copies would exist, in its entirety. This appears as if the same handful of posters are flipped time and again, and while some copies (of any poster are), not all will be. Since no one knows for certain how many copies of
Any poster truly exist (and this is very different than
"Known To Exist"), to put a concrete, finite number on one is a guess or opinion at best, regardless of how rare it is described to be.
Even the 4 copies of the Style F
Dracula OS are described as "only known copies." The Berwick Discovery showed that there are always others out there, potentially waiting to be re-discovered.