There is a limit of three plates in each pattern per customer.
Purchases are limited to three of each plate per customer.
この場合、次のような言い方ができますよ。
ーThere is a limit of three plates in each pattern per customer.
「おひとり様、各柄3皿まで」
each pattern で「各柄」と言えます。
ーPurchases are limited to three of each plate per customer.
「おひとり様、各皿3枚まで」
each plate「各皿」を使っても言えます。
ご参考まで!