The National Center for Missing Adults has a PDF listing at my count around 7000 names.
The National Center for Missing and Exploited Children has an Excel file with what look to be thousands more.
Possible explanations for each person on the list:
- they were never missing
- they've been found and haven't be delisted or those orgs haven't been informed
- each list is full of dupes and those dupes have been propagated to other lists
- they voluntarily disappeared
- they're still awaiting rescue
- they're dead and haven't yet been found or will never be found