Multiple graphics cards in a single system is frowned upon with vMix. There's a few posts over the years on this subject.

Utilizing the 4 outputs and 2 fullscreen outputs in (4K and Pro versions), that really leaves you with a ceiling of 6 outputs in that respect. Obviously, the 2 fullscreen outputs utilize your existing graphics card outputs. Getting the 4 other outputs out somewhere is where it gets a little entertaining.

You could use the external outputs by way of something like a Decklink Duo card, but that will only cover 2 of the 4 outputs at most. You could enable NDI on those outputs, but then you need another system to take those in and output to more monitors or get several NDI to HDMI or SDI converters.