ISA C8X is a 26-in/28-out audio interface, meaning it can send and receive up to 26 and 28 channels of audio respectively over the USB connection. There is a discrepancy between this number, and the physical I/O on the device, you can find a full breakdown below:

Software Playback: 26 channels

Inputs:  8 Analogue, 2 S/PDIF, 16 ADAT, 2 Loopback = 28 channels

Outputs: 12 Analogue, 4 mono headphones channels [L+R x2], 2 S/PDIF, 16 ADAT = 34 channels

It is important to note that there are settings and limitations which affect how many of these channels can be used in practice.  For example, the Digital I/O Modes and Sample Rate will affect the availability and channel numbering of digital inputs and outputs:

• Dual ADAT mode allows the ISA C8X to use all 16×16 channels of ADAT inputs and outputs, but this comes at the expense of the S/PDIF channels that cannot be used.
• Dual band sample rates [88.2/96 kHz] doubles the data that each ADAT audio signal, meaning the maximum limit is 8×8 ADAT channels in Dual ADAT mode.  Software playback is limited to 18 channels.
• Quad band sample rates [176.4/192 kHz] will disable all digital input/output channels and limit available software playback to 10 channels.

To achieve the highest possible channel count the ISA C8X will need to be in Dual ADAT mode with a sample rate of 44.1/48kHz.  This allows 8 analogue inputs, 12 analogue outputs, 2 Loopback inputs, 16×16 ADAT inputs/output and 26 software playback channels.

See the following section of the User Guide for more info on Digital I/O modes and channel numbering: Setting the digital port modes