GMAT Sentence Correction (SC) | Re: Nobody knows exactly how many languages there are in the

Nobody knows exactly how many languages there are in the world, partly because of the difficulty (of? ) distinguishing between a language and the sub-languages or dialects within it, but those who have tried to count typically have found about five thousand.

Got confused as to what 'it' refers to.

So started understanding the second half of the sentence.

The structure would be somewhat like this:
Nobody knows................but those who have tried to count typically have found about five thousand.

Subject
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