Carole,

 in English we can say either ‘ I found six of them ‘ or ‘ I found six ‘. 

Adding  ‘of them ‘ is an optional emphasiser. 

Both of these English expressions would be translated as ‘ J’en ai trouvé six ‘ in French, as either the pronoun ‘ en ‘ must be used, or the relevant noun repeated at the end of the sentence. 

‘ J’ai trouvé six ‘ is incomplete in French.

In reverse, ‘ j’en ai trouvé six ‘ can be translated to English either with or without ‘of them ‘ being included.