For anyone else looking into this, it's pretty hard to tell from this post how many people the source (Credit Suisse) estimates have negative net worth. Looking at this article
https://www.credit-suisse.com/us/en/about-us/research/resear... they estimate that "an estimated 9 percent of adults globally are net debtors". Together with their claim "we disregard the relatively small amount of wealth owned by children on their own account, and frame our results in terms of the global adult population, which totaled 4.8 billion in 2016", that's ~425 million adults with net debt.
It's hard to determine for which p Credit Suisse believes that the poorest p proportion people/adults have zero net worth. I tried to compute the implications of these claims (from the full PDF form of the report linked to above): "According to our estimates, half of all adults in the world own less than USD 2,222, and the bottom 20% of adults own no more than USD 248. The average wealth of people in these slices of the distribution is correspondingly low: just USD 159 for the bottom 50% and minus USD 1,079 for the bottom 20%." I believe these claims imply 42% All in all, that means that they estimate the poorest 2 billion adults have combined zero net worth (in fact, slightly negative). If you combine that with their estimate of 2.7 billion children of "relatively small .. wealth", that's about 4.7 billion people with approximate combined zero net worth. It's unclear how to properly account for children (maybe you shouldn't!), but you can easily decrease the number of adults to compensate. I think it's safe to say that (the people behind) this Credit Suisse report believe that the poorest 4.5 billion people in the world have combined nonpositive net worth.
(Note that the "people" numbers are higher than Oxfam's because they include children. Oxfam's "3.5 billion poorest people" is a reference to half of the world's population, even though Credit Suisse was talking about half of the world's adult population.)