If you you don't read the (manufacturer's) limitations, there are none.
If I had done the math, I wouldn't have sold 600 20x30 to 30x40 prints to a local client, Most from 12 MP captures many from 10 MP and even 6 MP. About 30 prints were up to 40x60 and were from scans, most quite low, but for those, they were supplied, not crafted by me.
On a previous job (140 prints), most were 20x30 and from 6-12 MP. One of the client's favorite images I ran 24x36 from a 4 MP jpeg (I was new to the camera and was shooting jpegs at an event when the sky opened up back when 1GB cards were in the $600 range). It was just fine. Close up (18-24 inches), I could tell it was from a low-res file. The client couldn't and nobody in the past six years has ever commented that this image may have been from a 4 MB jpeg...
With a properly captured image and great craft, the sky is the limit in reasonably good quality large prints from the D800 and even lower-res cameras. The best way to figure this out is to ignore the math and simple give it a try!
Now rip out that page telling you that you to stop at 16x20. You won't miss it with nearly 500 other pages!