In Patch 2.0.2, you will only need 2 
Resplendent Sparks to craft 1 random Mythic Unique. However, crafting specific Mythic Uniques is gated behind purchasing the Vessel of Hatred expansion!
In the latest Vessel of Hatred Developer Update, Blizzard revealed some important changes coming to the Mythic Unique Crafting System!
You will only need 2 Resplendent Sparks to craft a random Ancestral Mythic Unique at the Blacksmith. However, 2 Resplendent Sparks + a set of runes will create a specific Ancestral Mythic Unique, letting you craft the one you need. Given how the Rune system is tied to the Vessel of Hatred expansion, target-crafting of Mythic Uniques requires you to buy the Vessel of Hatred expansion. The crafting of specific Mythic Uniques is done at the Jeweler now.
