Not an enormous amount or anything, but a number that's probably much larger than you think it is. I only mentioned byuu because he's one of the more notorious hoarders in the hobby, including both game dumps and the revelant materials that accompany them (boxes, manuals, etc.), but he's not even close to the only one.
You get all kinds of excuses from them, ranging from "concerns over legal action" to "maintaining the 'purity' of the dumps" and all other bullshit in between. It really all boils down to they have it and you don't, and that was the actual goal that motivated them. Not any sense of preservation or community.
Some of their cheerleaders (>>4446693
for example) will occasionally show up and parrot tired rhetoric about "people deserving nothing because they didn't contribute" or "you're just mad you couldn't do the same thing first."
Ignoring the fact that these suggestions are as nonsensical as they are disingenuous, they do little but serve as a reminder of just how petty some people can be. Not everyone, like the hoarders and their cheerleaders would like you to believe, but some people. The hoarders didn't contribute anything, either. They just managed to show up at the right place at the right time with the right amount of money. From there, they made the conscious and deliberate choice to be a hoarding dickbag. The rationale that everyone is just as bad is bullshit. If that were really the case, there wouldn't be a single damned ROM out there to begin with.