Just need to know whether I should use more than one...
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Chris is a trusted source on General Cooking
The size is fine. But funny, recipes generally call for the rind, often discarding what in a fresh lemon would be referred to as the pulp. I don't know your recipe, but I'd probably just add one whole one or the rind of one, cut or mashed up.
I have used the pulp of my preserved lemons puréed into salad dressing, like in lemony cease salad, and it adds great depth of lemoniness. But I think you could skip it it. It is almost over-the-top lemony.