As of 2008, Apple sells 2.5 million Macs a quarter[1], which gives 10 million Macs per year. There's no way to sell 10 million units of a device that doesn't do anything your existing devices can't do. If the "tablet" is actually just a touch screen MacBook, then this makes sense.
Apple sold 3.05 million Macintosh® computers during the quarter, representing a 17 percent unit increase over the year-ago quarter. The Company sold 10.2 million iPods during the quarter, representing an eight percent unit decline from the year-ago quarter. Apple sold 7.4 million iPhones in the quarter, representing seven percent unit growth over the year-ago quarter. -- http://www.apple.com/pr/library/2009/10/19results.html
[1] http://www.computerworld.com/s/article/9135747/Apple_laptop_... (Extrapolating from one quarter isn't very accurate, but I'm lazy.)