Does subwoofer qty affect port size?

[rockFord_Expedition]

Cone area does, so yes.

[Triticum Agricolam]

So what is the port area that you have in your most recent SketchUp model?

As others have said, as you add more subs you need to add more port area. Assuming you have 1.2 cu ft of net volume tuned to 35 hz and are using 1200 watts of input power (what the subs are rated for) I would use about 24 sq in of port area. This is more than the 4 sq in per sub Sundown recommends, but even with this amount you will probably get some significant port compression. 24 sq in is about as much as you can use but still keep the port length somewhat reasonable.