請問 how many people came這句的問法是對的嗎？ 不是how many do people came嗎？ 或是came後面要放過受詞 有動詞came不是要放助動詞問嗎？ 還有要怎麼判斷what跟who是當主詞還是關係代名詞？

"how many ...?" 答案是一個數字 (number) 或一個量化的形容詞 (determiner)

"How many people" 成為問題的主語 (subject)，其答案也是主語。所以 "how may people came" 這句的問法是正確的。

例如：How many people came to the party last night? (答案：Twenty people came to the party last night.)

"how ...?" 答案是一種方法或情況
例如：How did those people come to the party last night? ("those people" 是主語，要放助動詞問 did ...come)

還有要怎麼判斷what跟who是當主詞還是關係代名詞？
1. 當主詞: 放在句子的開頭
例 What comes up must come down.
"Who" cannot be used as the subject of a sentence.

2. 當關係代名詞 (relative pronoun): 放在句子的中間，一個名詞後面
"what" cannot be used as a relative pronoun.
例 There is the man who lent me ten dollars yesterday.