Good question - I am not sure. I'm guessing that of those who own these types of places, there are quite a few people who own multiple of them.
Out of curiosity I went to a web site called StreetEasy that has NYC apartment listings. I was able to sort by single family buildings at $30MM and up and there are currently 59 on the market publicly. I'm guessing some of these are fishing expeditions and won't get anywhere near that, but the flip side is that there are likely some that are not publicly listed.