By rough calculation, each 144,000 BTU/hr stoker should be providing enough heat to boil 17+ gallons per hour. That ignores inefficiencies, and involves some rounding to not ignore them quite that much. Depending on how well you insulate, guesstimate perhaps 15 gallons per hour worth of heat - whether that's anywhere near what you actually get out of it probably depends more on the pan design and overall insulation. How big is the whole rig/pans going to be? My first guess would be that one should suffice, but most folks are prone to cranking the heck out of things (and blowing a lot of useful heat up the stack) so that would lead you to the road of overkill. If you are going to overkill the front end, do yourself a favor and design some extra length in too, so you get more use out of all that coal you'll be burning before you send it up the stack. Might also want to take extra care to make sure the coal smoke and syrup never meet - woodsmoke can be an acceptable "note" in syrup flavor, but I don't think coal smoke works the same way.
Two turkey pans on cinderblocks in the 1970's
4x5 no-baffle stainless pan, built sugarhouse ~1980 - buckets and snowshoes. 17 gallons in best year. Went off to college, nothing for 25+ years.
Thinking about getting going again in new location, in a small way. •• Cats, Coffee, Chocolate...Vices to Live By.