How many bricks would you need to build a chimney 12' tall 4' wide and 3' thick built from old brick?

That's (4+3+4+3) * 12 = 14 * 12 = 168 square feet of wall. At a standard of 6.86 bricks per square foot that would come to a little under 1200 bricks, but it definitely depends on the exact size of your old bricks.