How many? Is it rude to order just 2 and save room for dessert? Do you order an appetizer before them? Would you recommend small plates at all? Do they all come at once?
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I think there is no rule for this. Probably restaurants that serve this way vary greatly. From my experience, I appreciate being able to taste more things, so I enjoy it. Really, it's like having an assortment of appetizers. Look at other tables to see how much food is served on a 'plate,' or ask the waiter for some guidelines. You know your appetite and hunger level, so order accordingly. And save room for dessert? Sounds good to me!