One save :)
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New Game + is a must. Going full harem route: Use the wiki. DO NOT be afraid to turn down girls for hot coffee after dates, in fact you ...it's the only way to succeed as you need the girls to have very high love and obedience to form all the harems. Save often and don't be afraid to loose a week.
Then, you will have about 20 potential mothers. So that is every woman in a game has at least two kids. Doable.