I didn't know one person could produce this many works in their lifetime.
この場合、次のような言い方ができますよ。
ーI didn't know one person could produce this many works in their lifetime.
「一人の人が人生でこんなに多くの作品を作れるなんて知らなかった」
to produce this many works で「こんなにたくさんの作品を制作する」
in one's lifetime で「人の人生で」
ご参考まで!