Question

how does the amount of DNA remain constant through each new generation in a combination of DNA copies of 2 individuals

Answer 1

During S phase Dna is replicated in both the parents and one copy of each DNA Is inherited to 1 cell after meiosis 1 so after further division of cell in 4 cells dna from each parent is
Half in each cell ( half the genetic material or nucleic acid ) thus restoring the chromosome number with equal amount of dna

Answer 2

As the offspring receives one DNA copy from each parent, this complex mechanism helps to maintain the amount of DNA constant in an individual.

During sexual reproduction, the reproducing cells or germ cells have half the number (amount) of chromosomes and DNA as compared to somatic or body cells or non-reproducing cells. As the offspring receives one DNA copy from each parent, this complex mechanism helps to maintain the amount of DNA constant in an individual. If the DNA had to get doubled during sexual reproduction, then each generation would have double the amount of DNA content as compared to the previous generation. That is why the amount of DNA does not get doubled during sexual reproduction.

Hope it helps...