In ch.3, we are told that "Lady Lucas quieted [Mrs. Bennet's] fears a little by starting the idea of [Bingley] being gone to London only to get a large party for the ball; and a report soon followed, that Mr. Bingley was to bring twelve ladies and seven gentlemen with him to the assembly."
As it turns out, there are only two ladies and two gentlemen in the group besides Bingley, but I just wanted to point out that around 20 people seems to be what JA meant by a *large* party. I want to mention this because it comes up again later in the novel. ;-)