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[...] i decided to buy the cheap fireplace with my [...] I already had a cooking hearth and was curious if when i baked bread I could bake one grain for my fireplace, and one for my cooking hearth. Or if I can only bake one grain per turn.
With the baking bread action you can break as many bread as your ovens/fireplaces/cooking hearthes/etc. allow you.
As already mentioned fireplace and cooking hearth have no limit on how many bread you bake with one action (other than the two ovens for example).
Therefore you could with one cooking hearth you could bake as many grain as you want in 3 food each with one action.
If you own a Stone Oven and a fireplace you could bake up to 2 grain to 4 food each with the oven and aditionally unlimeted more grain to 2 food each with your fireplace all with one baking action.
Edit: A valuable recourse is the Unofficial Agricola compendium File on Boardgamegeek: The Unofficial Agricola Compendium