As of the Week Ending 19th September 2010 Encore was listed on the Billboard as this:
EMINEM - ENCORE - 5,200,921 - 19th Sep 2010
US: 5.2m
Europe: 2.6m
Japan: 432K
The maximum it has sold worldwide is 12m. Looking at the major music markets above it's most likely 11-11.5m. Out of all his studio albums Encore is the one that is being purchased the least right now so its unlikely to ever change.