Some achievements require you to perform 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.
To activate the log file, first edit My Documents\My Games\Sid Meier's Civilization 5\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.log.
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.