Wait... I am confused. We are talking about a DAW are we not? This is why back in my first comment I asked for his Interface. If the Equipment that he plans to use needs a x16 Lane or 2, then how many lanes are definitely within the scope of this.This is clearly well above the scope of the OP. If anything you are just confusing the issue.
OP, dont worry about PCI lanes, it's not an issue.
That said, most likely it won't be an issue, again this is why I said to be mindful of the hardware you are using. We don't know the OP's expertise, or equipment, but if it turns out he plans on using a couple UAD-2's or other Interfaces that require Multiple PCIE Lanes, it is a valid example. The Major point being, make sure you know what equipment you are using, and the requirements they have, in some High End cases, that does mean needing enough PCIE Lanes.