could you please tell which soundcard you are using?
What you describe sounds a bit strange as S/PDIF only provides 2 channels for each output (true for optical and coach outs). This would mean 8 optical & 8 coax outs on your soundcard(?).
My guess: in some cases the same optical out can be parametrized to behave as ADAT out. In this case up to 8 channels are possible. If your soundcard has two if them available up to 16 channels would be possible. If this is what you are talking about you‘d need an ADAT to audio converter. F.e. you could use a behringer ADA8200 if you‘re on budget (<200€). This converter makes available 8 additionally I/Os.
But as stated above: only a guess. With additional information I can probably be of better help.