There is still one spare slot on the grid after the US GP team dropped out.
So in theory, two more new teams could join.
However, the exorbitant cost of entry and the inability to fight with the existing teams on a level playing field (due to the changes in mid-season testing, etc.) put off too many prospective entrants, in my opinion.
Although I assume using Cosworth engines is no longer a requirement, as Mosley forced Caterham, Marussia and HRT to do, thereby crippling them immediately.