What stepper motor is in a CDROM drive?
Here's an example of a typical sled motor http://robocup.idi.ntnu.no/wiki/images/c/c6/PL15S020.pdf Note that it only does 20 steps per revolution. The small size and low cost of a CDROM motor mean that you can't expect high resolution from the motor. That is provided by the lead screw.

Also, not all such motors do 20 steps/rev. 24 is also a common spec. You'll just have to test your motor to see what it does, but that should be pretty much the ballpark number.