I recently completed Standard Ethanol. I'm a little confused about the role extra corn, rice and wheat play in the running of the corp. Why do I need more than one of each? Is there a way to get more than 5 executives, too? Thanks!
A city with Standard Ethanol produces 2

for each corn, rice, or wheat the empire has. You don't especially need "more than one of each"; 3 corn, empire-wide, is just as good for Standard Ethanol as one of each of those food resources.
Most corporations behave in a similiar fashion, providing some benefit in terms of hammers/food/culture/science for each relevant resource you have (but also consuming more gold if you have more resources, albeit that the conversion is always relatively efficient - you would normally be worse off if you didn't have Standard Ethanol and instead generated

with the slider). Ethanol and Aluminium are special in that they also add a strategic resource, oil or aluminium. Civ Jewellers is special in that it makes money as well as eating it up - I don't know, off the top of my head, if you can make a loss on Civ Jewellers.
I guess I've never used the real strength of standard ethanol, and I've always considered Mining and Sushi or Cereal Mills stronger (whichever you have more resources of) and these are not to be spread to other civs. Then the way Ethanol competes with Sushi and Cereal Mills isn't really a plus.
Mining Inc isn't really part of the question here - it can equally be had with Ethanol. But the real strength of Ethanol is quite clear. It goes like this:
1) I don't have Oil.
2) No-one will trade it to me.
3) I would prefer not be curb stomped by Tanks and Destroyers.
A similar deduction applies to Aluminium Co.