We only pass on half of our genomes to each of our children. Yet it would be wrong to believe that we could pass 100% of our genes simply by having two kids. It is obvious that the second child will never inherit exactly the other half from the first kid. If they did, they wouldn't even be full siblings, as they wouldn't have any inherited DNA segment in common from at least one parent.

In practice I believe that it never ever happens that two siblings inherit exactly the opposite 50% from either parent. That is because of the way meiosis, the process of chromosomal division and recombination, happens. Chromosomes have an elongated X shape (except the Y chromosome of men). Each chromosome has two long arms and two short ones. During recombination, most chromosomes get split in two, but some are inherited in a single block, which will be identical to one of the grandparents. Because each chromosome can be split almost anywhere along its chain of tens of millions of characters, the chances of having two kids with exactly the same split on even one chromosome are one in tens of millions. The chances of having exactly the same split on all chromosomes are almost nil (one in quadrillions).

So how just many children does one need to have to be sure to pass one every part of each of their chromosomes ? This is a very difficult question to answer because the whole process is so random and is purely a matter of luck. What is certain is that two or even three is not enough. With a lot of luck four or five may be enough. But in some cases we could imagine that even with 20 children, some small segments on some chromosomes will still be missing, while others will be passed on in almost every single child.

I don't think that a lot of people are aware that a certain percentage of their DNA will never be passed to posterity.