I have been told that you always use 'beaucoup de':
I understand the below should use 'de'
J'ai mangé beaucoup de sandwichs (I ate lots of sandwiches)
However, I was told even when you are saying 'lots of the sandwiches', you still use 'de', which I am not satisfied with:
J'ai mangé beaucoup des sandwichs que ma mère a preparé (I ate lots of the sandwiches that my mum made)
Should 'de' be used in the above or can you use 'des' in a case when you are referring to specific things?