Sony has sold 400m Walkmans; 50% are cassette-based models

"The first Walkman was produced in 1979. The picture shows the TPS-L2, the world’s first portable (mass-produced) stereo, which went on sale in Japan on July 1 that year and was later exported to the US, Europe and other places. Sony says that they managed to sell over 400 million Walkmans worldwide until March 2010, and exactly 200,020,000 of those were cassette-based models."
Source:  Crunchgear, 22nd October 2010