The most recent Survey of Consumer Payment Choice estimates that there are 178M card holders. The 2009 Survey of Consumer Finances (2) estimates that percentage of card holders of "bank-type" (eg: not Macy's) cards that revolve balances is just under 50%. With store cards, this number is right around 100M.

(1) http://www.bos.frb.org/economic/ppdp/2009/ppdp0910.htm (2) www.federalreserve.gov/pubs/bulletin/2009/pdf/scf09.pdf