The top 12,000

Fatih Guvenen and Greg Kaplan, drawing on IRS and SSA data, write (pdf):

"So far in the 21st century (2001–2012): 
IRS and SSA data reveal diverging patterns in top income shares – the IRS data show a steady increase, whereas the SSA data show no increase at all. The difference is due to the increasing importance of income accruing to pass-through entities (partnerships and S-corporations), which is included in the IRS measure of total income but not in either the IRS or SSA measure of labor income. 
Moreover, the bulk of this growth in income from pass-through entities was concentrated at the very top of the distribution – above the 99.99th percentile, a group that contains only about 12,000 households. The share of incomes above the 99th percentile (around $372,000 in 2012) but below the 99.99th percentile (around $7.2 million in 2012) has barely changed in the last two decades."