I agree with what you and Ann have said. If you look at what goes before and after Mrs. Bennet's words, I feel she is taking offense to Darcy's implications that the country is rather a hick place, not sufficient for the studying of character as may be done in the city. From the beginning Darcy, has looked down on the people here, seeing them as beneath him. Even though Mrs. Bennet does not seem to have tuned in to the entire gist of the conversation, I feel she is right in picking up on Darcy's statement that is, I think, subtlely criticising the lack in number and variety of people in the country. What could there possibly be to study is his way of thinking. All are the same: country, country and more country with no class and so few at that. Mrs. Bennet quotes a number of four and twenty families which she think is considerable but is obviously to the Netherfield party, slim pickin's. :-)