How many unique addresses own LAND or Estates?

Official DCL AMA Answer: (19.11.2018)

“Etherscan, unfortunatelly, is not reporting the original 4000 land holders. When we deployed the land, the ERC721 standard was in draft and it changed significantly since that time. The fingerprint of the Transfer events before September is different. All the data is in the Ethereum blockchain, feel free to dig around. You might need this to guide you through it! https://github.com/decentraland/land”