I am sure I am missing something when Mrs. Bennet says, both in the novel and in the movie that they dine with four and twenty families and this is found to be hysterically funny. (snip)

The laugh is on Mrs Bennet not because this is too few but too many. The supposition behind Darcy's remark is that in a country district there would be few families worth associating with, and here is poor Mrs Bennett admitting that they 'dine' (and this shows a degree of social intimacy, unlike simply calling on someone) with such a large group. And it was quite a lot, when people were so limited geographically in their socialising. In Jane Austen's letters, for example, it is clear that her family only socialised with a few other families who lived nearby. So Mrs Bennett is amusing because she has shown herself undiscriminating and vulgar.