It's not an arbitrary number - the post limit for a level 50 player is a maximum 20 posts a day, as detailed here: http://forum.square-enix.com/ffxiv/t...-%28Aug.-22%29

As they say, 'knowing is half the battle' - it shouldn't be too hard to keep count of the amount of posts you've done, but having some kind of counter on the screen wouldn't be a bad thing.