I'm not sure about that, Wrench. Because I have seen quite a few static aircraft with num decals (since I had made only 16 or so), so I'm a bit confused regarding the 'cycling back'. I do know it that the decal numbers start over after '099', but not after the skin has run out of decals. I definitely need to check this. 
  
EDIT: Did a quick test, removed the numbers.lst (The skin had 12 modex decals, num000-011) 
 
... 
 
 
  
According to what you said there (assuming I fully understood that   ) Cowboy 23 should have the Modex number 600 (num00), since Cowboy 22 had the last decal in this series (num11), but it doesn't. That's why I'm convinced that the decals are not cycled back before reaching #99. 
  
2nd EDIT: re Spitfire decals 
There's nothing wrong with the method Wrench described, but the disadvantage is that you would have to do 16 skins (with decals & textures) in order to cover the Wings you mentioned. Why not cover the 16 units with a single skin? 
 
The squadron letter (DW) is a Level 1 decal, the individual aircraft letter (F) is a Level 2 decal and the serial number is another level 2 decal. The insignia is part of the texture. Thus you have a single skin that can be used to represent any Spitfire squadron that was present during the BoB.