Is the sentence " "There were hundreds of students, not much of whom were girls" correct? Can we use much instead of many in these structure.
"Much" is a word used for non-countable items. These items include things like fluids or non-physical substances; an example would be milk. I cannot have 5 milks or 10 milks, I just have milk.
Since girls can be counted (you can have 1 girl or 100 girls), you should either use "not many" or "few". So, two possible ways of writing the sentence would be:
"There were hundreds of students, not many of whom were girls."
"There were hundreds of students, few of whom were girls."
I would personally prefer to use "few" in most cases.