A woman's eggs are fully formed and stowed away in her ovaries from before birth. Each mature egg contains one copy of each gene in the human genome—half the amount necessary for life. (Each sperm also contains a single copy of each gene in the human genome.)
The maximum number of eggs that a woman will ever have is the number she has when she's a 20-week-old fetus. She'll have about 7 million of them then, 600,000 when she's born, and about 400,000 at puberty. Once a woman hits puberty and menstruation begins, her ovaries release one of those eggs every 28 or so days.
During each cycle, even though multiple eggs start to develop, hormonal signals ensure that only a single egg will be released and the other eggs will regress (it's not evolutionarily wise to blow them all at once, so the body gives females a 30-or-so-year window in which to conceive).
When we are 40 there are about 400 left.
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