Post independence of India there was a focus to reduce the population in order to curtail poverty. However I am interested to know what scriptures say about number of children one should have, and explanation (if any). Did the number vary by varna ?
Many sons should be desired as says one of the verses given below. The reason for the instruction is also stated along with.
With the very birth of a son, a father is freed from his ancestral debt and attains to purification on that Very day, for he (i.e., the son) liberates him from the hell. (54)
Many sons should be desired : for if any [of them] would go to Gaya, or celebrate a Horse-Sacrifice, or dedicate a Neela Vrisha* (55)
All the departed Mane's, dreading hell, desire for "the son, who would go to Gaya and become our saviour." (56)
Atri Smriti (or Samhita).
Gaya, mentioned in the verses, is considered as the holiest Tirtha for performing ancestral rites in Hinduism.
Ancestral offerings is one of the basic and one of the foremost duties of every Hindu. It is recommended to be offered on the Krishna Paksha (the waning phase of moon).
The various rewards that are obtained for offering ancestral offerings on the various 15 days of the said Paksha is mentioned in Apasthambha Dharma Sutras 2.16.7-22 and they are as follows:
7 No matter what day of the fortnight of the waning moon it is offered, it gives delight to the ancestors. The specific reward earned by the performer, however, depends on the time that he offers it. 8 If he offers it on the first day, his children will turn out to be mostly girls; 9 on the second day, his children will not turn out to be thieves; 10 on the third day, his children will be eminent in vedic knowledge; 11 on the fourth day, he will become rich in small animals; 12 on the fifth day, his children will turn out to be boys, and he will have a lot of offspring and not die childless; 13 on the sixth day, he will be adept at travelling and gambling; 14 on the seventh day, he will be successful in agriculture; 15 on the eighth day, he will become prosperous; 16 on the ninth day, he will acquire one-hoofed animals; 17 on the tenth day, he will be successful in business; 18 on the eleventh day, he will acquire iron, tin, and lead; 19 on the twelfth day, he will become rich in cattle; 20 on the thirteenth day, he will have many sons and friends, and his children will be beautiful but die young; 21 on the fourteenth day, he will be successful in battle; 22 and on the fifteenth day, he will become prosperous.
So, having many sons or offspring is considered as a good thing, a reward. Therefore, having many sons/offspring is desired and considered as advantageous as per scriptures.