how many times do you need to be reborn to succeed impregnation ?
Not sure what you are asking so will give an overview of how things work.
The girls have a fertility cycle, they are most fertile for 2-3 days during their ovulation week (this is randomized at startup, but they then stick to their 4 week cycle),as the player you have a maximum amount of sperm you can have at one time (increased by buying capacity upgrades from Gabrielle) and you also have the maximum amount that can be regained during one time block (increased by buying regen upgrades from Gabrielle). Every time the player cums (from handjobs, blowjobs, sixety-nines, and sex) he ejaculates 60 % of what he has stored.
For quickest impregnation:
Focus on fucking the girl you want to impregnate in the morning and night (without cumming in the afternoon or evening) during their risky days (displayed in the girl’s stat page). Buy upgrades from Gabrielle to increase regen and capacity, this will be the best way to get her pregnant.
Edit: I am looking at possibly making some way to increase the girl’s fertility in Gabrielle’s shop, however this is only an idea at this point as I have not thought of a consistent way to implement this. If this is added it will likely not be for a while as it would be added during the shop rework, this is currently planned for around 0.2.1.