中译英:"他是第几个进入房间的？“

哇!我终于找到正确的答案了!就是这个!
is he the first to enter the room? 看!
汉语中有些对序数词提问的问题，译成英语时要改变提问法，
如：
1.这是你第几次来中国？
Is this your first visit to china?
2.你们队得了第几名？
Did your team win the championship?
3.这孩子是你的老几啊？
Is this your oldest/youngest child/son/daughter?

How many people had there been before he came into the room?

it is hard to translate this directly to english.

here is some common way to asking this question.

"How many people came in the room in front of him?"
"When did he came?"
"Is he the first people in here?"

when did him come into this room
what is the order of his coming this room

He is the first of several entered the room