Minecraft wiki entry for Smelting, associated text: If there are fewer items than the fuel can burn, some of the fuel will be wasted. Accordingly, you will usually want to smelt things in multiples of 8 (when burning lumps of coal or charcoal), or occasionally 3 (2 wooden planks). For individual smelts, you can use sticks or saplings, 2 of which smelt 1 item. If your fuel source is plentiful, such as charcoal later in the game, you may not care about fuel wastage, so items can be smelted in any number.