In 1999, the U.S. Mint started producing a series of 50 "state" quarters, one for each state in the United States. They are scheduled to come out at a rate of five per year over 10 years. In other words, a new one is issued every 10 weeks or so. They are being released in the same order that the states joined the union. To see a complete schedule of when each state's quarter will be issued, see this page.
The mint publishes production figures on this page. These figures indicate that the mint produces something like 300 million to 350 million quarters every month. Therefore, in a 10-week run, each state gets something like 800 million or so quarters minted. These quarters are dumped into general circulation by sending them out through the commercial bank
These links will help you learn more: