I think there's only really two real sets of artwork - the EGA and VGA releases. Some (EGA running on CGA or Hercules, VGA running on EGA) dither down the artwork rather than there being extra artwork for there, and the other versions are largely derived from whichever of EGA and VGA was a better fit for the target platform. Still a meaningful amount of work (especially reducing the 256 colour VGA art to 32 colours for the Amiga port), but it's the kind of thing that happened in games porting in that era.