「人山人海」英文用「a sea of」表示！
(Image from freepik.com)
你喜歡賞花嗎？中文常用花海來形容百花齊放的壯觀美麗，英文也是用「sea 海」這個字來形容「大量的…、滿滿一大片的…」，說法為「a sea of sth」：
a sea of sth 大量的…
The whole field was a sea of yellow flowers between May and June.
I looked out and saw a sea of people at the concert.
一望出去，演唱會現場根本人海茫茫！
She was surrounded by a sea of paperwork.
她被堆積如山的文件包圍著。