Yes, it count's towards the +12 max to an ability. You can still add +12 to other stats, but you can only add an additional 10 with a Dark Elf.
As a further note!!! sub-Race Ability bonuses do NOT count towards the requirements of spells or feats! If you are a paladin or Cleric Aasimar, the +2 to wisdom does not count towards the required amount of Wis needed for cleric and Paladin Spells, nor would Half Giant Str Bonus help towards feats like Devastating Crit or Overwhelming...