The correct answer is 12, but what's odd about Ninio's Extinction Illusion, named for French scientist Jacques Ninio, is that it is impossible to see all of the dots at once. I tried tilting my head 45 degrees and still could not perceive all of them in a single glance. I also tried backing up and looking at the monitor from across the room. No dice.
"Researchers' prevailing theory…is that we have more neurons clustered at the center of our vision than the outside. The lack of cells in our periphery renders us nearly blind to things far enough outside the center. To compensate, the brain takes its best guess at what's out there based on the more-visible gray areas. This makes a dot seem solid when we're looking right at it but invisible when viewed with a sideways glance."
That's a fascinating description--it means our brains are essentially rendering, incorrectly, areas outside of our direct area of focus.