I have no idea. I think some people are talking about Vivi being a certain numbered straw hat or something. Nothing to do with Carrot.
Oh ok.
Well then I'll put in my 2 cents here.
Vivi is officially labeled as Straw Hat 5.5. Oda has made very clear effort to make sure that no one has forgotten about her in all the years that she has been absent from the main cast. Vivi gets random cover pages all the time just like the front and center members.
I wish he numbered Karoo though, he's not chopped liver. He gets pages just as much Vivi, he's probably like 5.6 or something.
There is a balance within the crew as far as strength goes. And while it's completely relative, Usopp is supposed to stay the weakest. Even during her time with the crew Vivi wasn't even as capable of as Usopp. Now that 2 years have passed she would now be a burden to the crew more than ever.
Her role is that of a diplomat, one with influence and political power. Her contributions to the crew will be in the form of hidden agendas when she's making propositions that secretly(or maybe even openly) aid in what ever the Straw Hats are trying to achieve. She is the legitimate Vergo of the Straw Hat crew.
Then we have the number 6. That goes to Chopper for sure while the 6th person goes to Robin. Neither of which got their X'th person chapter. I would only use that button six as foreshadowing on Nami's back pack if the crew and Vivi were all headed to the same place right away. Right now Vivi is going towards Mariejois, and while they may meet up in the middle, it's most likely they won't see each other again until the Reverie arc which will probably happen after the next major immediate events.
If anything, if I saw a random 6 on Vivi, I'd assume that was alluding to Robin getting her 6th Person chapter. It would be a Jinbe/Nami type of dialogue only between Robin and Vivi. Robin getting her chapter after settling her guilt with Vivi would be worth of a 6th Person chapter I think.
So that 6 button? I doubt it's anything but a button and a fold of the pack.