I had no idea that there were this many expressions.
ご質問ありがとうございます。
「こんなにたくさんの」は英語で「there are this many」と言います。
また、「こんなにたくさんの表現があるなんて知らなかった」という文脈で、そうすると、「I had no idea that there were this many expressions.」になります。
この場合、文章は過去形なので、最初おしえたの「there are this many」から「there were this many」に変更しました。
ご参考になれば幸いです。
I didn't know there were so many different expressions for this.
I hadn't realized there were that many different ways to describe this.
この場合、次のように言うと良いですよ。
ーI didn't know there were so many different expressions for this.
「これに関してそんなにたくさんの表現があるとは知らなかった」
I didn't know there were so many ... で「そんなにたくさんの…があるとは知らなかった」
ーI hadn't realized there were that many different ways to describe this.
「これにはそんなにたくさんの言い方があるとは知らなかった」
I hadn't realized there were that many ...「そんなにたくさんの…があるとは気がつかなかった・知らなかった」
ご参考まで!