Dear student.
The reason why female produces only one egg is:
Egg cell is large, more the number of the egg cell will lead to overcrowding and womb will not be able to carry it. Handling more than one egg is difficult, to avoid difficulty, the ovary releases one egg every other month.
During a normal menstrual cycle a woman has just one dominant follicle (regions of ovary which produce egg) and releases just one egg during ovulation or menstrual cycle. But in abnormal cases, the two follicles become dominant in women and women releases two eggs in one cycle. These two eggs are fertilized separately which result in the formation of fraternity twins.
Regards.