How many were not owned by the person turning them in?
Will the rightful owner(s) ever have them returned?
When guns are reported to the police as stolen, they are entered into a national database, the NCIC.
When guns are turned in, they are run through the NCIC to see if they have been reported as stolen.
Lots of old guns do not have serial numbers; many people do not report guns that have been stolen; so the NCIC database is far from comprehensive.