Terri - according to the website catholic dot hierarchy dot org, there are 17 Cardinals listed for the USA.  Other than those named in my earlier comment who are under 80, the name below are all over age 80.  I don't know why the last two are on the list as U.S.A cardinals - I don't think they ran a U.S. diocese or archdiocese.  Don't know if others are born in USA, but check out the names:

William Keeler	Baltimore
Bernard Law	retired from Boston, now Roman Curia
James Stafford	Denver
Adam Maida	        Detroit
Roger Mahony	Los Angeles
Justin Rigali	        Philadelphia
William Levada	San Francisco, retired from Curia 
Theodore McCarrick	Washington
	
Agostino Cacciavillan	   once papal nuncio to US
Lubomyr Husar 	   Archbs Emeritus of Kiev, Ukraine

to AnonAJ's point.  If the U.S. has 7% of Catholics, 7 cardinals out of 120 means the U.S. represents 5.8% of voting cardinals; 8 U.S. cardinals would be 6.7%; 9 U.S. cardinals would be 7.5%.  Francis could "fairly" appoint 1 or 2 more.