I've personally heard of four, in my range of acquaintances of acquaintances. And if that's true for me, that's probably true for most people. I don't even really associate with sketchy people (though I do know some people from Rotterdam), and I've not been looking for them either. I know one of my friend's brother's (now ex-)girlfriend's sister went, etc. Such stories do the rounds all the time. So my guess it's at least a couple dozen for the Netherlands alone, extrapolating to the rest of Europe you'd be in the hundreds easily.
Of course, it's not like there's a news article on every one of them, so I can't substantiate. But it definitely used to be a thing. (ISIS is pretty much done for, and everyone's being kept a strict eye on nowadays, so it's not like anyone's still going.)