12 puppies are common with the mastiffs. I've had several litters with that many. My mentor had a bitch that had 18 last spring! She was worn ragged having to bottle feed constantly! When I have more than 6 I supplement bottle feed.
Oh yeah, one thing I was going to mention but forgot - green discharge BEFORE the first puppy is usually a sign of distress, that means the placentas are detaching. Green discharge AFTER the first puppy is normal. Watch for large amounts of blood, that could signal a uterine rupture. I think that labs usually have pretty smooth whelpings, so you are probably not going to have issues.