How many sticks does an armor stand take?
It takes 6 sticks and 1 smooth stone slab. One log already yields 8 sticks, so 2 are left over.
Does the slab have to be smooth stone?
Yes. Smooth stone comes from smelting stone, which comes from smelting cobblestone, which is why the slab costs 6 smelts for 3 cobblestone.
How much coal do I need for the armor stands?
One armor stand takes 1 coal, and eight take 2 (12 smelts). The count comes from the slab smelting, not the sticks.
Can I use less cobblestone for several armor stands?
It already uses little: one round yields 6 slabs from 3 smooth stone, so 3 cobblestone cover up to 6 armor stands. For 8, it is 6 cobblestone.