From an article on ICV2:
Mayfair Games sold around 370,000 copies of the Settlers of Catan English language base game in 2013, a Mayfair spokesperson told ICv2. We asked because in a New Yorker profile of game designer Klaus Teuber, the magazine reported that Mayfair had sold over 750,000 Catan-related products in 2013, and we were curious how many of those were base games. The answer is roughly half.
Over 18 million copies of the game have been sold worldwide to date.
Mayfair hopes to sell 500,000 copies of the Settlers of Catan base game in 2014, the spokesperson said.