Last night I played my first game of Agricola and I was amazed how great game it was.

But one thing is not clear to me, when owing Fireplace / Cooking Hearth, can I trade for example 2 x vegetable for 4 food (or for example 3 sheep for 6 food) in one Fireplace ? Or is it allowed to trade only one vegetable / animal for food ? And can I trade vegetables and animals at the same time ?

When I played with my friends yesterday we played so that we can convert as many vegetables/animals for food as we can. But when I reread rules I've noticed line saying: 

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> A player may own several Fireplaces and Cooking Hearths.
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So why would I build more Fireplaces / Cooking Hearths if I can convert as many vegetables / animals to food in my turn ?