From the sheer context of the post I can assume that the Soul Stone will be one per character as it's a reward for Platinum tier, this can also be backed up by the fact that there is no evidence to the contrary and the fact that the reward is the item itself and not a recipe. Whether it's bound to a character or if it's trade able makes a huge difference! No doubt someone can get multiple toons and get three stones then further more upgrade a Mythic/Arcane item with them....

The real question is:

If an item can be crafted with more then one Soul Stone?
Can these Soul Stones be traded?
If they can't be traded can an item upgraded with one still be traded?
The Soul Stone has an orange name, does that mean getting it would be a Mythic Achievement?