I found this approximate answer here: http://www.newyorkfed.org/aboutthefed/fedpoint/fed01.html "There is about $829 billion dollars of U.S. currency in circulation; the majority is held outside the United States." A couple of paragraphs later, it says the figure is from December 2007.
The bank replenishes physical currency and coin through the Federal Reserve Bank.
federal reserve
Yes
All current circulation bills are Federal Reserve Notes so depending on the dates you're interested in you can simply look in your wallet. Older dates of Federal Reserve Notes (before 1999 depending on denomination) and all silver certificates are pretty much out of circulation though, so you'd need to look on eBay, go to a coin and currency show, or visit a dealer.
Federal Reserve System
Federal Reserve Banks distributes new currency for the Department of the Treasury, which produces it. Depository institutions buy currency from Federal Reserve Banks when they need it to meet customer demand, and they deposit cash at the Fed when they have more than they need to meet customer demand. Most medium-sized and large-sized banks maintain reserve accounts at one of the 12 regional Federal Reserve Banks, and they pay for the cash they get from the Federal Reserve by having those accounts debited. Some smaller banks maintain their required reserves at larger, "correspondent," banks. The smaller banks get cash through the correspondent banks, which charge a fee for the service. The larger banks get currency from the Federal Reserve and pass it on to the smaller banks.
According to the Federal Reserve website (see related link below) there was approximately $1.15 trillion in circulation as of November 14, 2012, of which $1.11 trillion was in Federal Reserve notes.
Over the long term, the major factors affecting member bank reserves are Federal Reserve credit holdings, holdings of international monetary reserves and currency circulation. Additional factors, which do not change greatly over the longer term are Treasury currency outstanding, Treasury deposits, and foreign deposits at Reserve Banks.
I'm assuming it's The Federal Reserve
The Purpose of the United States Federal Reserve Board is that of a governing body to oversee and make important decisions to do with the Federal Reserve Bank and the United States' currency and monetary issues.
currency notes
The ones who controls how many of the dollars are in circulation. "Give me control of a nations currency, and i care not who makes the laws" or something like that, is a famous quote from one of them folks from The Federal Reserve