"The ovaries are two small organs... located in the female pelvis. They are attached to the uterus, one on each side, near the opening of the fallopian tube." Each ovary contains "about 1-2 million oocytes," or eggs, at birth and that is all that the female will ever have as they do not produce more. What remains after puberty is maybe a few hundred thousand and will most likely be depleted by the time a woman reaches menopause.
Each month, hundreds of eggs start to mature in preparation for fertilization. While that seems like a large number, most eggs will not reach full maturity and die in a process called atresia. Of those that do survive only one will be released, sort of how one egg needs just one sperm to be fertilized.
Now, because of the limited supply of oocytes a female has, when she becomes pregnant her ovaries are blocked from receiving the hormones FSH and LH which stimulate oocyte growth and release. This ensures no more oocytes will be matured or released until after childbirth.