Logarithms of negative numbers aren't a thing, and so, technically, if your liabilities exceed your assets (i.e. you have a negative net worth) you shouldn't appear on this index. But Bloomberg has added those people -- which could be students with debt, after all -- into the -2 category of their scale. These are people with a penny to their name.
Now, the number of adults in each bracket is purely an estimate. If you look at different sources, you will end up with different numbers. Bloomberg believes that there are 2,800 adult billionaires in the world (numbers 9 to 11); whereas Credit Suisse's estimate is about 1,600. (I wonder if it's easier to estimate the number of billionaires or the number of -2's.)