Why can't many which in this sentence be replaced by many?

How to translate this sentence and what grammar is used? Please explain in detail, thank you.

The answer is as follows: the above sentences are non-restrictive attributive clauses, and the clauses must have relative pronouns to connect the main clauses. The relative pronoun =books can be changed into a compound sentence.

Our library has a large number of books, many of which are in English.

Our library has a large number of books, many of which are written in English.

If many of them are complex sentences, they are not unrestricted attributive clauses.

Our library has a large number of books, many of which are written in English.