The ovulation occurs at around mid cycle. (e.g- in a 28 day cycle ovulation occurs at around 14 day) so after the menstruation
(starting of a new cycle) you should focus the 2nd and 3 rd week and should try regularly. However it does not mean that you will be 100% pregnant if you try in those days but it gives you best probability to be get pregnant. Because several other factors comes in between. But if you get your luck a single try would be enough to be get pregnant. so just keep trying.. BEST OF LUCK.