The pronoun "many" in the passage refers to "companies" in the sentence: "Companies used advertising to sell their goods throughout the country."
What does the noun mean
In the given passage, the pronoun "many" is used to refer to the subject "companies." The sentence preceding the pronoun talks about how companies used advertising to sell their goods nationwide.
The word "many" is then used to emphasize that a considerable number of these companies employed catalogs and mail orders to reach people in rural areas.
This suggests that numerous companies utilized this method of selling goods, demonstrating a widespread practice among businesses during the period of industrialization in the United States.