"The bureau oversees 264 million acres of federal lands in the West -- an area nearly the size of Arizona, Colorado, New Mexico and Utah combined. Those lands are home to an estimated four million to five million archeological sites -- and more than 20 million artifacts have been collected there. "

Extrapolate this then:
Map references: North America 
Information courtesy of the CIA.
Area: 
total: 9,629,091 sq. km 
land: 9,158,960 sq. km 
water: 470,131 sq. km 
note: includes only the 50 states and District of Columbia
1 square mile = 640 acres = 2.590 square kilometers 
Per BLM article above:264 million acres \ 4 million sites = approximately 66 archaeological sites per acre
That sounds rather high, lets cut it down to 1/4th that amount.  66 / 4 = 16.5 sites per acre.

Question:  Wherever an artifact is found is an archaeological site isn't it?
9,158,960 sq. km \ 2.590 sq. km = 3,536,278 square miles total land area of U.S.A.
3,536,278 square miles * 640 acres in one square mile = 2,263,217,920 total acres 
2,263,217,920 total acres * 16 archaeological sites per acre = approximately 36,211,486,720 total archaeological sites scattered throughout the continental United States of America.

Granted, not all areas contain the same density of archaeological sites, in fact, some areas have a much higher distribution of sites, especially Paleo distributions in the Southeast, but overall, an rather interesting extrapolation.

Does the name "Carl Sagan" ring a bell, anyone?
I could go on and on. 

Example:  How many archaeologists are currently in the active work pool?  Average number of artifacts recovered from sites.  Average number of years devoted to a specific excavation?  Projected future work pool and increased excavation\report timelines. 

We must save these billions of archaeological sites and related artifacts for future generations, which might not even be remotely interested in such mundane items, when they will be existing in a far different environment than most of us common folk can even imagine in our wildest dreams.