The same number versus different number distinction is often theoretically and ethically important but is rarely clear-cut when it comes to questions of policy. This is because many policy decisions affect both who comes to exist and how many people come to exist: both the constitution and the size of the future population. For instance, vigorously discouraging teenage pregnancy may well result in different children being born, but also in fewer children being born.
The second, more complicated, distinction is between choices that are identity-affecting and those that are not. Some choices alter the characteristics of a determinate future person (making someone more or less healthy, for example) whereas others amount to choosing between the creation of distinct possible future persons (choosing a healthier one over one with a genetic disorder, for example). Selective reproduction (or at least the kind of selective reproduction) involves the latter rather than the former.
The choices that we make when we practice selective reproduction are identity-affecting. They are also existential, causing people to exist who otherwise would not, or preventing the creation of (possible future) people. To make this distinction clearer, think about the following actions or policies. If the population contains a number of women like Grace, it is safe to assume that implementing policies that discourage smoking during pregnancy will have identity-affecting side-effects.