Yes looks like DeWitt started off looking for twelve and relied on Lucretius to come up with that number. I think he's making reasonable presumptions, and in fact most of what he comes up with lines up with Diskin Clay, but certainly not everything.
As usual we're largely on our own as to what is reasonable to presume and what is not. Almost like we're similar to Theon in "A Few Days In Athens" -- piecing things together over time.