I/O channels, or racks?
Each rack can have 4, 8, 12 or 16 modules.
You can have (with high density) modules up to around 250+ channels of data.
The rack can hold a controller or Ethernet Brain, the controllers can have up to 16 charts or programs running to manage their I/O or other racks.
The EB's talk back to another controller, or you can address them via another language like .Net or C# etc.
It would be more than worth your time to review the PAC System Guide;
So back to your question.... Lots. Lots and lots. Thousands +. Just depends on how you chose to configure the system.