It's impossible to say how many lurk.
Trolls who delurk are shown the door in short order.
Every now and then, one delurks for honest debate and gets his clock cleaned. This used to happen more frequently, but has diminished over time to the point that it hasn't happened in a long while.
We've had a few conversions, over the years, but I think the main thing they've learned is that they're massively outclassed and can't win an open debate, because at the end of the day, there's no genuine ambiguity to play with. History, the law, morals and ethics are all on our side.
The anti's case rests entirely on manufactured ambiguity. Without it, they have nothing to stand on.