You don't really require any. It is the biospheres that would supply the Agricultural Manufacturing Plant.So, how do you figure out how many refineries or smelters you would need to supply a given tonnage of Agricultural Manufacturing Plants?
I am not even sure that the rules provided even allow you to determine a sensible number of them since an Agri plant doesn't actually require a level of input feedstocks. It just makes a level of output for a level of plant tonnage requiring a level of Power and makes 25% more if your planet has an AG trade code.
You could do some clever maths to work out how many tons of Manufacturing plant produced enough common consumables for two people for a day as that is the level of output of a ton of biosphere, except that the biosphere does all that without the benefit of the Agri Plant (and the Agri plant does it all without the benefit of a biosphere - or materials at all).
All a bit vague unfortunately.
My preferred solution is to assume any manufacturing plant requires the same 50% (or 1D x 10%) rule for fabricator feedstock for all manufacturing plants (except for high tech products). 1 ton of Common Consumables will on average require Cr175 in "feedstock" to produce the Cr500 worth of output. This feedstock is not all foodstuffs of course since any product will need to be packaged. this might also be provided by the smelters.
Common Consumables of course could cover more than just food. You may prefer to use the cost of a meal as your "target" price and work out how many meals equate to a ton of output and calculate your inputs cost from that.