126 items, divided by 7 classes gives 18 items per class. There are 6 item types (helm, shoulders, chest, legs, main-hand weapon, off-hand weapon) for the heroes so 18 items per class gives three sets available to be used as transmogs.
At the time this was introduced, I think there were 16 sets per class, now there’s more. I asked, the week this functionality was released, what happened to the other sets being available.
In addition to the fact that this functionality only makes 3 of the sets available per class, if the hero makes use of one of these sets, they must make use of the base colouration / variation of the appearance, i.e. they cannot get a version that would be available due to resonance if the hero was actually wearing the pieces. Now, those appearances clearly exist, because every single piece of gear already has those resonance versions available if the hero’s wearing the item(s).
Every single set should be available to be transmog’d, and colourised by resonance.