One, though someone said they are pressure sensitive like the trackpad x,y, and z cordinates. Of course not much has been written for the third cordinate. The newer trackpads can actually divided into grids creating virtual buttons and or scroll space (side or up down).
The program SideTrack is a replacement driver for the trackpad (touchpad) found on Apple PowerBooks and iBooks. It brings many of the advanced trackpad features found on Windows laptops to MacOS X.
Below is a shareware program that allows for the addition of a second or third button virtually built onto your track pad(virtual grid) I use it on my ibook so I have 3 buttons and 3 scroll areas, pretty cool.
SideTrack supports these features:
Vertical scrolling at left or right edge of pad.
Horizontal scrolling at top or bottom edge of pad.
Map hardware button to left or right click.
Map trackpad taps to no action, left click, left click drag (with or without drag lock), or right click.
Map trackpad corner taps to mouse buttons 1-6 or simulated keystrokes.
Compatible with uControl, including uControl's scroll emulation.
This public beta version of SideTrack will expire on June 30, 2004.