24 cars last I heard.
Microsoft has also announced a Forza Motorsport 2 demo coming May, 2007 featuring a total of 24 cars with full Xbox 360 Wireless Racing Wheel support. However, car modification is certain to be disabled. "There is no point in decking out your ride and then not being able to save the design or upgrades after you exit the demo." The demo will not feature multiplayer features over Xbox Live, although split-screen multiplayer was neither confirmed nor denied.[10] The track being featured in the demo is Mugello Short.
Confirmed Demo Car List to Date (With More to Follow)
* 1997 BMW Motorsport M3 E36
* 2006 Chevrolet Corvette Z06
* 2003 Dodge Viper SRT10
* 2003 Dodge SRT4
* 2004 Ferrari F430
* 2005 Ford GT
* 2005 Ford Mustang GT
* 2002 Honda Integra Type-R
* 2005 Lamborghini Gallardo
* 2006 Lexus IS350
* 2004 Mazda RX-8 Mazdaspeed
* 2005 Mercedes SLR
* 2003 Nissan Fairlady Z
* 2007 Porsche 911 Turbo (997)
* 2005 Subaru Legacy B4 2.0 GT
* 2005 TVR Sagaris