If that's on the box. If this can't reasonably be expected and is hidden in an FAQ somewhere, not obvious when purchasing, then a box that says 12 eggs should contain on average 12 eggs. Dunno to what extent variability is allowed (when not purchasing huge amounts); for discrete items like eggs probably not but for gram weights usually it's +/- some amount.
Edit: found a comment relevant to allowed variability https://news.ycombinator.com/item?id=34802080