Yes, they both mean many. 'A' is used when you want to give an example or speak generally, 'the' is used when you are talking about a specific thing (not any of the others). They don't affect the meaning of the following words though. 'Large' does suggest that there will be more than just 'a number of'. When you say 'a number of' you are talking about a section or a part of something, not the whole. 'The large number of' is talking about all of something.

Yes. This is why some victims disappeared completely.