Around 44.4 million
This answer is derived from information provided at http://www.chron.com/disp/story.mpl/business/realestate/5985260.html
The article claims that
a) there are 4,000,000 mortgages considered to be "at risk"
b) this represents 9% of the mortages in the U.S.
A quick calculations shows that there are 44,444,444 mortgages
Obviously this is a very rough estimate