By 1999, Sony had managed to sell 186 million cassette players, according to stats supplied by the company. But the cassette was quickly getting eclipsed by other technologies, most notably the CD but eventually the MP3. And, since 1999, Sony has shipped a more modest 14 million* cassette players worldwide, hardly the type of bullish growth that characterized the 80s and 90s.
Here's a sales breakdown from the late 90s, offered by Sony on the cassette Walkman's 20th anniversary. Those were better times, and we're guessing this type of breakdown was offered less and less as Sony started to falter in the 2000s.
*we initially reported 220 million players; the cumulative is actually closer to 200 million.