Did you take that photo in the cemetery area recently? Those stones in your photo don’t appear to be the headstones that are by the mansion right now. They are probably headstones for other individuals. In the below photo from 1910 you can see just how many headstones there were in the cemetery. The vast majority of them were not moved–just the Berrian ones it seems.
Those two large headstones in the foreground appear to be the Dorcas and Samuel Berrian stones. Peter Ostrander recalls that the Berrian headstones were removed from the cemetery long ago. He says they have been on the grounds of the Van Cortlandt house (by the Sugar House window) for 20 years. Only very recently were they moved again to the back of the Van Cortlandt House offices when they started doing repairs to the Sugar House window.
Your photo does make me wonder just how many headstones are in the cemetery. They are usually covered by leaves and not that easy to see. It looks like you cleared some of that before taking your photo. I would like to go and do a complete count. Let me know if you want to join me.