So the One Piece world is estimated to be 2.7 times that of our Earth, to keep it simple 8 billion times 2.7 is 21.6 billion. This means that there are potentially 21,600 people with Conquerers Haki. Let's assume a third of them are either Marines or Pirates. Over 7000 marines or pirates could have it. So there is no problem with this. Of those 7000 maybe 700 have an advanced form. Seems like plenty of opponents to realistically have advanced conquerors.
