Out of the 200 million eggs a female oyster produces only a handful grow to adulthood.  

Oysters can produce up to 200 million eggs in a mating season, but only a few of these offspring will develop into mature larvae and even fewer reach adulthood. (The larvae are called "spat")

However, one oyster species, the Olympia oyster, doesn't release its babies until they have grown to be larvae with a protective shell. These female oysters hold onto the fertilized eggs for 2 weeks. After that, many of these oyster moms turn into males.