Analog mixer with a computer

If I have an analog mixer, 24 track, I plan to go from the mixer into a set of channel strip racks and then into the CPU from there.

I just don't understand how the audio is going to get from the computer back to the analog board or will it happen via the 2, 16 channel interfaces when I'm going to incorporate on my rack mount, I understand the software how to route each individual fader to a different output, but I'm wondering if I need to buy a total of 4, 16 channel rack strips one for input and one for output or is 2, 16 channel interfaces good enough for sending and receiving Audio through each individual fader.
What interface(s) are you considering? No, you don't need to buy 100 of everything, but some foresight and planning does need to go into it. Do you *need* an analog mixer, or would a control surface do?

Here's a few articles that might help: (CAVEAT EMPTOR-- Ethan Winer is very knowledgeable, but also a very controversial figure in audio production for a number of reasons)