The $12 trillion is the total of all mortgages; Fannie and Freddie hold some $5 trillion worth. But many of the other companies are in a dicey situation.

Today the government took over Indymac bank, a small one that was worth only about $32 billion. It will cost the FDIC (thus taxpayers) some $4-8 billion to secure depositors.
Others are in line to be taken over as 'unsound.'