It's a very subjective term and it's possible to use degrees of rarity perhaps - the SAAFs had 800 issued but most were destroyed, maybe 40 exist, collected and 'hidden'? The 'Grail' Speed, 376.0822, had about 1500+ made, but is 'rarely' seen for sale. EZM1 ZUZ had only 22 issued, the Sternzeits maybe about 50+, A386s about +/- 2500 iirc; all could be described as 'rare' or as you say 'scarce' to some degree, particularly when compared to main stream production numbers, but to be honest, if you are looking for one then you already know where it stands in respect to availability/production. When describing a particular watch you could add condition into the mix as well, the earlier type ('first edition') or better condition one that is all correct for example will make it both rarer and more desirable I suppose.
Rare seems an understatement for the one or two offs such as the prototypes though -
(Courtesy Herr Lang)
Or even when only a handful are known - like the other UG Cairelli, HA-1 -