「China has many people」 is fine grammatically and practically.
Trying to find fault in this sentence is like splitting hair.
One argument I can see is the age-old debate whether an inanimate object (such as 「country」), could 「possess」 something with the word 「has」.
However, it』s been long accepted that 「country」 in possessive form, such as 「country』s leader」, or「country』s population」 (2.2 million in Google search), is fine.
Having said that, I would say "China has a large population". Its purely a matter of style and I am not claiming that sentence is better than "China has many people".
这种用法在口语中很普遍,例如下面几句都是和老外聊天的时候对方用的:
Is Xian the city which has biangbiang面?
Yes, Britain has a lot of accents for sure!
I think airports and tourist attractions around the world tend to have thieves and scammers.
但是题目中那句,感觉有点怪怪的,如果是说 中国有很多人XXX,China has many people who XXX, 就更自然一些。如果就是说「中国人口众多」, 那么说 China has a large population 更好点。
他们一般都说「lots of」 或者「a lot of」,比如说「He has a lot of partners」而不是「He has many partners」,尽管后者语法完全正确。我问过一个英国人,为什么口语里不用many,他说因为很多人懒得区分可数还是不可数,many和much全都用「lots of」代替很方便。
而为什么用「There are a lot of people in China」,而不是「China has a lot of people」,原因在于前者更像是一句完整的句子,后者听起来像是话讲了一半。英国人会以为你要说「China has a lot of people doing sth.」 然后下半句那个「doing sth」等了半天一直没出来,发现你原来早就说完了,真是让人摸不著头脑,哈哈。
比如他们会说,"Barclays has a lot of branches opening on Sunday morning."(巴克莱银行有不少分店周日早上开门)这才是「xx has a lot of xx……」的常规用法。
当然最好还是说"China has the largest population"更准确一些,毕竟要强调的是「人口」这个词。