You can take as many things on this tray as you want, as long as the total is under one million yen.
・You can take as many things on this tray as you want, as long as the total is under one million yen.
「合計が百万円以内である限り、このトレーの上にあるものは好きなだけ持って行っていいよ」
as long as(〜である限り)と as many as you want(欲しいだけたくさん)を組み合わた、ネイティブらしく自然なフレーズです。「百万円」は 1 million yen と表現します。