Question

how does the amount of DNA remain constant do each new generation is combination of DNA copies of two individuals

Answer 1

the amount of DNA remains constant even though the new generation have copies of two individuals because during the transfer of genetic material some jeans are lost while some jeans are gained this makes an equal effect and there is no increase and decrease in the amount of DNA

Answer 2

during replication both DNA of parents gets half and then combine
for example in human being total number of cromosome are 46 in reproduction it becomes 24 when transfer of gamets takes place
DNA from mother is 23 and one sex cromosome and same happens in father's gamets
so 23+23=46
so amount of DNA remains same