Technically, one egg doesn’t equal a soul until it meets a sperm. At that time for about a faction of an instant it is one cell, however then it starts to divide right away. Identical twins, I believe (could be wrong) are formed within the first or first couple cell divisions when the cells divide and seperate rather than divide and stay together. This is what forms two individuals. In that fraction of a second I think God sorts out the souls as he sees fit.
Since an unfertilized egg is not a person, and a fertilized egg is, how the sould comes into being is a bit of a mystery.
Good question though, I’m adding it to my list to ask if and when I get to heaven.