I think [NumAura()] should work for debuffs.
eg. [NumAura("Arcane Buffet"):RED]
[HasAura("Arcane Buffet")? "BUFFET! x " NumAura("Arcane Buffet")]
Should show the text 'Buffet! x 5' for a stack of 5 etc...
Works really nicely. Just tested by duelling a mage with the improved scorch debuff and it works like a charm :)
Just need to edit the text zone to make it work with Lady Sacrolash's Dark Touched and Grand Warlock Alythess' Flame Touched =)