The U.S. has 3,091 active landfills and over 10,000 old municipal landfills, according to the Environmental Protection Agency. However, in the "good old days," every town (and many businesses and factories) had its own dump. There are 39,044 general purpose local governments in the United States - 3,043 county governments and 36,001 sub-county general purpose governments (towns & townships). One suspects that there are many more old and abandoned commercial, private, and municipal dumps than the 10,000 estimated by the EPA.
All landfills could require remediation, but particularly landfills built in the last 60 years will require thorough clean-up due to the disposal of highly toxic chemicals manufactured and sold since the 1940's.