The number of channels it can take can be any number from 64 (I've never seen less) to 65,535.
It all depends on the options ordered with the radio when it was purchased originally, or if it was upgraded later.
The only programming option for it is the Harris ProGrammer or RPM software packages. Which are about 300 dollars for conventional only models, and about 2500 dollars (maybe less if you can find a dealer that discounts) for the trunked version.
It's likely that you only need the conventional version of ProGrammer. It should not be too hard to find a dealer that will sell that to you but getting the full trunked version is a bit tougher.
There are no other options for programming the Orion. You use ProGrammer, or you use RPM. RPM being newer and built to support newer radios in addition to the legacy products. And it requires upgrading the firmware in the Orion to be compatible with RPM so really it's not something you even need to worry about. You want the conventional version of ProGrammer.