I just hit 50 today on my White Mage and I started organizing my gear sets as I plan to (eventually) have all classes maxed out, this includes DoH and DoM.

In total there are fifteen available gear set 'slots' and there are a total of nineteen classes (twenty if you count the Sumooner and Scholar jobs)

So, SE will need to implement more gear set 'slots' to address this. Obviously not everyone wants/needs to max everything, but for completionists like myself, I feel this will become necessary very soon (if it isn't already)