It varies by manufacturer, but most seem to regard more than one as acceptable. As said, location on screen, and proximity to each other also considered.
each pixel is made from 3 transistors, and at 1024 x 768 that it 2,359,296 transisitors. for one pixel to not work right, all it takes is for one of the 3 transistors to fail, and that is a failure rate of 0.00004%
Still annoying, but unlikely to be a reason for a warranty claim.
But we recently bought 8 LG 17" TFT's at work, and not one dead pixel to be found.