My question is how much items can one piece of coal smelt in a furnace. Self explanatory title.
One piece of coal/charcoal supports up to eight furnace operations... provided the input slot of the furnace is continually loaded. [If you try to smelt 4 iron ore and 4 gold ore by adding the gold ore as the last iron operation finishes, you will find the smelting of the fourth gold ore does not complete. ]
Note that the whole of the piece of coal is expended even if only one operation is performed. (Hence the tendancy to smelt in groups of eight or use alternate fuels – eg. two saplings will fuel one furnace operation – as well as to use hopper arrangements when using coal blocks [80 operations per] or lava buckets [100 operations per]. qv https://minecraft.gamepedia.com/Smelting#Fuel_efficiency )
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
I suppose it might be a fallacious argument (ad absurdium?), but because of recursion you can actually smelt 57 items on a single piece of coal. The trick is to first burn 7 logs to make enough charcoal to handle the remaining load.