This hadees has came from many chains of narrations. This is also a متفق علیہ hadees, which means it is present in both Sahi Bukhari and Sahi muslim. And interestingly some of the chains does not contain any number at all.
It was narrated that Jaabir ibn ‘Abd-Allaah (may Allaah be pleased with them both) said: “The Prophet (peace and blessings of Allaah be upon him) asked me, ‘Have you got married?’ I said, ‘Yes.’ He said, ‘A virgin or a previously-married woman?’ I said, ‘A previously-married woman.’ He said, ‘Why not a young girl, whom you could play with and she could play with you?’ I said, ‘I have sisters and I wanted to marry a woman who could gather them together and comb their hair and take care of them.’ He said: ‘You will reach, so when you have arrived (at home), I advise you to associate with your wife (that you may have an intelligent son).’”
(Narrated by al-Bukhaari, 1991; Muslim, 715)
According to another report narrated by al-Bukhaari (2257), “… so she could teach them and discipline them.”
Now the same hadees with the number of sisters mentioned is :
It was narrated that Jaabir (may Allaah be pleased with him) said: “My father died, leaving seven – or nine – daughters, and I married a previously-married woman. The Messenger of Allaah (peace and blessings of Allaah be upon him) asked, ‘Did you get married, O Jaabir?’ I said, ‘Yes.’ He said, ‘A virgin or a previously-married woman?’ I said, ‘A previously-married woman.’ He said, ‘Why not a young girl whom you could play with and she could play with you, and you could laugh with her and she could laugh with you?’ I told him that ‘Abd-Allaah had died and left behind daughters, and I did not want to bring to them someone like them. So I had married a woman who could take care of them and discipline them. He said, ‘May Allaah bless you,’ or similar kind words.”(Narrated by al-Bukhaari, 5052)
So, As a brother has mentioned in comments that this is the number reported by the narrator(it means he does not recall definitely what number was that), it does not mean that Jabbir R.A was confused about number of his sisters.
And on a side note this is another example of how perfectly preserved ahadees are, and how careful narrators were in the matter of narration. Even the narrators mentioned that he was not sure about number of sisters.