To make 12 Eyes of Ender in Minecraft, you need 6 blaze rods.

Quick Scoop

Each Eye of Ender is crafted from:

  • 1 blaze powder
  • 1 ender pearl

Each blaze rod can be crafted into 2 blaze powder.

So the math is:

  1. You need 12 blaze powder for 12 Eyes of Ender.
  1. 1 blaze rod → 2 blaze powder.
  1. 12 blaze powder÷2=6 blaze rods12\text{ blaze powder}\div 2=6\text{ blaze rods}12 blaze powder÷2=6 blaze rods.

In other words: farm at least 6 blaze rods for the crafting itself, and a couple extra if you want spares in case Eyes of Ender break while you’re finding the stronghold.

TL;DR:
You need 6 blaze rods to craft exactly 12 Eyes of Ender.

Information gathered from public forums or data available on the internet and portrayed here.