Question

How does the amount of DNA remain constant though each new generation is a combination of DNA copies of two individuals ?????

Answer 1

When the fertilisation takes place after the the sperms and the egg fuse together the development of the zygote begins during this period the dna from the sperms and the egg is copied into the zygote . due to meosis the number of chromosomes remain constant in the species . Haploid male gamete (n) + Haploid female gamete (n) = Diploid zygote (2n).
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Answer 2

this is because at the time of gamete formation the number of chromosomes of each individual are halved.. thus when the gametes combine the normal set of DNA is restored again..
let 2n be the normal DNA in each species.. then n will be in gametes.. thus 2n in the zygote..