A lot of etc

Hi
Can someone help me with this?
Determiners such as ‘a lot of’ and ‘a few of’ etc., can be pronouns, but for the life of me I cannot understand why?
The oxford dictionary tells me that in this sentence “there are a lot of actors in the cast” that ‘a lot of’ is a pronoun, but it also tells me that it is synonymous with many. If I check the usage of ‘many’ in this context I believe it is an adjective???
Thanks