There were at least two rumours about the large party that Bingley was bringing to the Meryton Assembly, and neither of them were true.

To quote the passage more fully:

and a report soon followed, that Mr. Bingley was to bring twelve ladies and seven gentlemen with him to the assembly. ... instead of twelve he had brought only six with him from London -- his five sisters and a cousin. And when the party entered the assembly room it consisted of only five altogether -- Mr. Bingley, his two sisters, the husband of the eldest, and another young man.

Clearly Mr Bingley did not bring 12 ladies and 7 gentlemen. Neither did bring 6 people, nor did he bring a cousin. So I think both of those rumours were false. The narrator also tells us that he brought "his two sisters" NOT two of his sisters, which indicates that two sisters is all he has.