How many cities in the world have subway systems?

Question

How many cities in the world have subway systems?

Answer 1

NYCGuy10 left out Camden, NJ

Answer 2

Based on a book I have "Transit Maps of the World", I would estimate that there are about 70 or so cities that have systems that can be called subways/metros (this does not include light rail or streetcar systems).

Of that 70, United States cities have a large share (12 )

New York  (NYC Transit)

Newark, NJ (PATH system)

Boston  (MBTA)

Philadelphia (SEPTA)

Philadelphia (PATCO)  

Chicago  (CTA)

Washington, D.C (WMATA) - technically not a city but a district

Baltimore (Metro)

Los Angeles (Metro)

San Francisco (BART)

Atlanta (MARTA)

Miami (Metrorail)

Edit:  I fixed the above list (in respone to the answer below). The PATCO system between Camden, NJ and Philadelphia, PA is considered based out of Philadelphia not Camden just as PATH is considered based out of NJ even though it provides service between NJ and NYC.