How many months after first baby are you most fertile?

My husband and I have a absolutely wonderful little 3.5 month old and we want more kids. We want to start trying around 6 months BUT bc I am breastfeeding I haven't got my period so I can't track my ovulation, any mom's know what number month is best for conceiving? It's it 3-4 , 4-5, 5-6, 6-7.. and so on. I just don't know when we should start trying since I habe no idea my ovulation window whike still bf.