Just a quick FYI
Some achievements require you to preform a task a number of times (like buy 1000 tiles, or chop down 1000 forests). A complete list of all those numbers can be found in a log file, along with anything else you want to know about achievements (it is the source of most but not all the information in these posts).

To activate the log file, first edit ""My Documents/My Games/Civilization V/config.ini", and change the line LoggingEnabled = 0 to LoggingEnabled = 1. Then restart the game, and it will create the log file. Go to "My Documents/My Games/Sid Meier's Civilization 5/Logs and open achievements_debug.txt"

The log file is created every time you launch the game, and is not updated until you restart the game. So typically, it will not show the most current stats.

Full Article Here[forums.civfanatics.com]

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