The numbers aren't public yet, but it's probably low. I don't think they are dumb enough to expect this device to sell anywhere near the Galaxy S10 or an iPhone.
It may not be a huge sales success, but apparently it's above Samsung expectations.
My suspicion is that it's people interested in the engineering of the screen rather than consumers who've identified a personal use case for it.
Whether or not it ends up truly being practical, it's a phone that looks and does things that are radically different from most mainstream phones.
People will think their fancy future folding phone is cool, and other people will probably see it and think it's interesting and want to talk about it or see it work.
Kind of like paying $600 plus a 2-year contract for an iPhone in 2007 with a slow processor, no 3G, no MMS, no copy-and-paste, etc.