You can use either for ‘many”, but of course this lesson was specific to quelques, plusieurs and de nombreux, and does not cover beaucoup de, although it would be useful to add it.

After my post yesterday to which Cécile responded that it was a matter of style, and on which Chris also commented, I have checked further with Robert - the definition of “beaucoup” starts with use before a noun - with “beaucoup de” meaning ‘une grande nombre de’.  It is always worth knowing more than one way to express things - and it is rare that a language only has one. Larousse, Robert and wordreference all confirm that “beaucoup de” is applicable, and give varied examples of its use with countable nouns. 

De nombreux is more limited in meaning, beaucoup de has wider application. 

https://dictionnaire.lerobert.com/definition/beaucoup

https://dictionnaire.lerobert.com/definition/nombreux

https://french.kwiziq.com/questions/view/beaucoup-de-many

Thanks Maarten.. You have answered my question but it does need to mention beaucoup de in the lesson