Most simply, a potential person could be defined as the currently existing genetic material that will constitute them, such as a sexually viable egg and sperm cell taken together, also when still being located in separate places.
[1] Potential people may also be defined from reproductive capability, which also includes the presence of other necessary factors for becoming a person,[1] such as the availability of a womb to grow in, the will and means of parents to conceive, or even the care after birth to raise the individual into a complete sentient being.
Actually, in this sense, for a man that intends to conceive only two children, the billions of sperm cells he produces throughout his lifetime may, taken together, still only be regarded as a contributing factor to a maximum of two potential persons.
It has been argued that the mere potential of becoming a person confers moral rights on a prima facie basis,[1][2] or by holding that they are really in some sense actuals.
For example, a remote friend has a baby, and is about to conceive another, and, for example, happiness is taken as the end-in-itself and receiving a toy is taken as an instrument to it, then, the yet unconceived baby may not be regarded as currently having ethic value, in contrast to the existing baby, but nevertheless, the instrumental value in the action of posting a toy to either of them can be regarded as equivalent, because either alternative would generate equal amount of intrinsic value in the form of happiness in the future, with some modification for, for example, the risk of failing to conceive again, and the burden for the post office or parent in storing the toy until, at least, birth.
[1] For example, if happiness is regarded as the end-in-itself, then, it has been claimed to be morally objectionable to bring a potential person into existence that is predicted to be very unhappy.
However, a major reason that has been given to be cautious about performing abortion with such motivation is the fact that the likelihood of successfully bringing the new child into existence is substantially lower, as the parents may separate, one of them may become sterile, or they may change their minds about having children.