Paul Bahn (he's a rock art specialist; his is the only book[1] I've read on it other than Diamond) estimated the population at 10K pre-contact, though he also theorizes there was an earlier Spanish contact than the recorded one which prompted a collapse of their belief system and tipped an already fragile society over.
[1] https://www.amazon.com/Enigmas-Easter-Island-Paul-Bahn/dp/01... -- cool book. The title is tongue in cheek; he mostly is annoyed by people who don't simply apply standard archaeology and anthropology to the island and look for more complex theories than necessary.
[1] https://www.amazon.com/Enigmas-Easter-Island-Paul-Bahn/dp/01... -- cool book. The title is tongue in cheek; he mostly is annoyed by people who don't simply apply standard archaeology and anthropology to the island and look for more complex theories than necessary.