Do all 12 cards show on your credit report and do all of them report the limit? To my knowledge there's no way for someone reviewing a credit report to see whether you have or have not "carried" a balance, they only know the most recent balance reported for each account. American Express can, of course, see the payment pattern on the accounts they issue.

A credit report will show if you have other types of debt (e.g. mortgage, installment loans), how old these obligations are, how typical they are for your age, occupation, neighborhood, etc.