Question

What are the possible consequences if only one egg is implanted

Answer 1

  are we talking human reproduction? in this case only 1 egg is implanted. in unusual cases there could be 2 eggs implanted meaning fraternal twins. if 1 egg splits, we get identical twins. with only one egg implanted, there is only 1 kid being produced. 

the female has eggs stored from birth. 1 ovary 'pops' once a month to either be discarded via a period or implanted after fertilization. multiple eggs released is usually the effect of fertility drugs making many eggs get released at once. this is unusual unless fertility drugs are in use