World seed number

Only if you haven't created a new world since then and kept the config file, then the seed number in WorldGenerator.eco will still be the same. Also keep in mind that the exact same world will only be generated if the whole WorldGenerator.eco is identical, a change to any variable can change how the world looks.

You can find the file in /Steam/steamapps/common/Eco/Eco_Data/Server/Configs.