Nearly 5,000 CONUS scenes were processed to create LANDFIRE 2010 (representing change 2008-2010). The latest update, LANDFIRE 2012, employed a new image compositing process that used over 20,000 scenes from 2010-2013 in CONUS alone. That is more than 5,000 scenes per year to create composites.
Further Reading:
+ Delivering data for a decade and counting
Scenes from the Polar Night
Landsat satellites have begun regularly acquiring images of ice at the poles during the winter, with enlightening results.