Two plates; two helpings or two servings (the quantity equivalent to two dishes/plates).
寿司を二皿お願いします。
Two plates of sushi, please.
彼はビュッフェで二皿分を平らげた。
He finished two plates' worth at the buffet.
パーティー用に二皿ずつ取り分けた。
I portioned out two plates for each (person) for the party.