Nearly 91.1 million Americans, 78% of households, had one or more credit card at the end of 2008. The average household holds seven credit cards, including bank cards and retailer-specific credit lines. Half of college undergraduates had four or more cards in 2008 and had an average balance spread between them of more than $3,100. Sources: Creditcard.com, MyFICO.com, Federal Reserve Survey of Consumer Finances. -- Reported by Joe Mont in Boston.