Who are first-borns? Only sons or also daughters? If the first-born of a family dies, does the second oldest take his/her place? Likely that sounds a bit odd, but in Num 3:22 below 1 month olds are excluded from the count in the census. Does that mean the first child living over a month is called the first-born even if 3 brothers and sister before him/her died before the age of 1 month? Example Numbers 3:40. Is the number first-born counted exactly the number of families? Or could the number of families be much higher because many lost child at toddler age?
ב"ה Shalom, The ramifications of a first born according to the Torah are in regard to the obligation to redeem him and that he takes a double portion in inheritance. A first born, refers only to a male first born who was the first offspring to the mother who bore him. If the first born, God forbid, dies, the one following does not need to be redeemed but he takes the place of the first born in regard to the inheritance. (Exodus 34:19, Numbers 3:40, Mishna Bechorot 7:1). The first born are only redeemed after 30 days when according to the Torah they are no longer considered non-viable children. All the best