I'll leave a more advanced explanation to somebody else.

Dead pixels are from a malfunction/breakdown in the pixel, often effected by heat, current, and time. So some of the dead pixels might need warm up, hot day, or high gain to be visible, but over time the pixel circuit breaks down even more so that the pixel becomes more obvious at less heat & gain. Eventually the dead pixels may all become visible at start up and no gain (though I suspect there is some sort of masking mechanism in the camera). You can expect more over time as well.

What we shouldn't expect is too much less dead pixels than a consumer camera (a 3 CCD version, as each pixel has three CCD pixels it is probably three times more likely to have one). But what you don't want is serious dead pixels, or any but the mildest ones in the main part of the view (middle, left, right) where actors faces normally are, especially the middle area).

So from previous experience, how many dead pixels do you guys usually find on the 500/5000 and other cameras?