Here’s a thought experiment to help you understand:

Let’s say ten unrelated couples, with different last names, land on an island with no other people. They follow the custom of the family’s name being the father’s name.

Each couple has two children. To make it simpler, we’ll say each child survives, is fertile, etc., no uncles marry nieces, and for the purpose of this example are heterosexual, etc.

Half the new children are males, half are females. So the next generation has the same ratio of ten males and ten females.

Now let’s say the sex of their children are randomly assigned.
Some couples will have one son and one daughter.
Some couples will have two daughters.
Some couples will have two sons.

The men with only daughters have just as many children, but they don’t pass on their last names nor their Y-chromosomes. Likewise the women with only sons will not pass on their mitochondria.

In both cases, the number of offspring is the same, every member of the population has descendants, but this one trait could be lost for each person.

If one family had two daughters, and one had two sons, but all the rest had one of each, then that next generation would have lost one mitochondrial line, leaving 9, and one Y-chromosome, again leaving 9.

You could repeat this, and with random distribution of sons and daughters there would be additional losses in both of these over the generations, even though everyone has the same number of living descendants. Eventually there would just be one. That one is the one that is the mitochondrial Eve. And there is a similar Y-chromosomal Adam.

Of course, this example is flawed by starting with a tiny population which would eventually be forced to marry/mate with cousins or other relatives, which would then complicate the math. In fact, there is a branch of math that deals with the complex nature of populations.

But this gives you the tools to understand the concept of mitochondrial Eve.

(It isn’t true, by the way, that there is a 100% chance that the mitochondria always vary measurably from mothers to daughters, hence the oft-repeated statement that “she had to have had at least two daughters, because if there was one one, then she would be mitochondrial Eve” is not absolutely correct.)

This is called the founder effect.