Firstly, the DHS survey final report usually does not provide the target population size estimation (women 15-49) which is the first piece of information needed for de-normalizing the women weight. So in general cases, you need to find out this piece of information from outside sources. But for the Nigeria 2008 DHS, we did provided an estimation of the target population size (women 15-49), as you cited 31,624,485, given in table B.1.
Secondly, for the number of women interviewed in a DHS survey which is the second piece of information needed for de-normalizing the women weight, we provide this piece of information in the DHS survey final report, usually in chapter one, table 1.2 in the section "Response rate". For the Nigeria 2008 DHS, this number is 33385. The number you cited 36800 from table B.1 is not the actual number of women interviewed, it was the expected number of women interviews calculated in the sample design.
Hope this helps.