Decoding Dolby Digital and DTS is generally done by a dedicated, application specific IC that has DSP processing and the DACs built into their design. The decoding itself is done by a DSP engine.

You don't need more than one DAC per channel of audio unless you are running differential output, which is something that is not done all that much. Just like you only need one power amplifier per channel, the same thing holds for the DAC. If somebody is using more than 2 DACs per channel (differential), I would have no idea what they are doing with them since their presence is not required to convert to analog.