I am running an ASUS Sage 10 X299 with an i9-10980xe, 128 gb of ram, two M.2 drives, four SSDs, a P4000 video card, 2 BMD 4K decklink monitor cards, one BMD Decklink 8K Pro, one Decklink Duo, and an ASUS thunderbolt 3 card.

I want to replace the decklink duo with another 8K Pro.

Will I have enough PCI lanes to do this?

Thanks in advance...

Your CPU has 48 PCIe lanes, double what almost all desktop chips of the previous generation had.

According to the Asus site the motherboard takes up 4 lanes for its own purposes (USB, audio, etc- this is extraordinarily common) leaving you with 44.

Slot 1 gets a dedicated 16 lanes. Put your GPU there. (Even so 8x is plenty enough for what you'll be doing in vMix).

The rest of the slots are 16x/8x+8x pairings. If you use the first of the pair only you get 16x. If you use both they automatically go into 8x each.

The M.2 slots will run at 4x each.

GPU 16x
M.2 8x total
4k monitor cards 8x total
8k pro 8x
Duo 4x
Thunderbolt card (I don't know what one you're getting but generally it's 4 lanes per port so let's call this 8x total)

Without the TB card you're already at 44. It doesn't appear that you can bifurcate (split the lanes) of slot 1 so there wouldn't be any reason to move the GPU elsewhere in an attempt to only give it 8x.

I'd recommend dropping the 4k monitors and the duo to get a second 8k pro.

Otherwise look at something like a Dell Precision 7920. It's workstation class and uses dual Xeons so you get PCIe lanes for days.

Edit: I just realized you were asking about adding another 8k pro. No, I think you're 4 lanes short without dropping another card.