It takes a woman nine months make a child, so assuming 1 month = 30 days...
OPTION A: 1 woman = 270 days
(Not feasible, not enough time)
OPTION B: 9 women = 30 days
(Possible, but on a tight schedule)
OPTION C: 27 women = 10 days
(Within feasibility and Saotome harem powers)
OPTION D: 90 women = 3 days
(Not feasible, requires Aijou Rentarou stamina)
OPTION E: 270 women = 1 day
(Death by Snu-Snu)
Citation: Elementary Mathematics