Clifford, I've read that there were 44 in all who died, but three died on Morris Island, and several died on Hilton Head, so the 13 you mention may be all who died at Ft. Pulaski. It does seem, though, that with the accounts of the suffering and dying, many more than 13 could have died during that terrible winter at Ft. Pulaski.
I'll keep digging.