Go to this website and watch the video at the bottom of the webpage: https://help.bitpay.com/businesses-and-organizations/getting-paid/what-are-my-settlement-options

1 BCT=100 million satoshi
21 million BCT= 2.1 quadtrillion satoshi
1 BCT=$1,000 per BCT
21 million BCT=$21 billion dollars
100 million satoshi=$1,000
2.1 quadtrillion satoshi=$21 billion dollars
You figure the rest.

If everybody is selling there bitcoins. Who is going to buy them?

I think your are wrong. It is not meant for investment. It is a completely digital money. It is the first decentralized peer-to-peer payment network that is powered by its users with no central authority or middlemen. From a user perspective, bitcoin is pretty much like cash for the Internet.

Partial quote form Bitcoin.org vocabulary webpage:

"Bitcoin

Bitcoin - with capitalization, is used when describing the concept of Bitcoin, or the entire network itself. e.g. "I was learning about the Bitcoin protocol today."
bitcoin - without capitalization, is used to describe bitcoins as a unit of account. e.g. "I sent ten bitcoins today."; it is also often abbreviated BTC or XBT.
Block

A block is a record in the block chain that contains and confirms many waiting transactions. Roughly every 10 minutes, on average, a new block including transactions is appended to the block chain through mining.
Block Chain

The block chain is a public record of Bitcoin transactions in chronological order. The block chain is shared between all Bitcoin users. It is used to verify the permanence of Bitcoin transactions and to prevent double spending."

HTML5 is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and current version of the HTML standard.

Wire transfer fees charged by the 10 largest U.S. banks:
Banks                    Domestic - Incoming   Domestic - Outgoing   Foreign - Incoming   Foreign - Outgoing
Bank of America               $15                     $25                           $16                            $45
Chase                       $15                     $30                           $15                            $45
Wells Fargo                       $15                     $30                           $16                            $40
Citibank                       $15                     $35                           $15                            $45
U.S. Bank                       $20                     $30                           $25                            $50
Capital One                       $15                     $25                           $15                            $50
PNC Bank                       $15                     $25                           $15                            $45
TD Bank                       $15                     $25                           $15                            $40
BB&T                               $15                     $25                           $18                            $65
SunTrust Bank               $15                     $25                           $30                            $50
AVERAGE                       $15.50                     $27.50                   $18.00                    $47.50

quote from U.S. DEPARTMENT OF TREASURY website about legal tender question

"Legal Tender Status
 
I thought that United States currency was legal tender for all debts. Some businesses or governmental agencies say that they will only accept checks, money orders or credit cards as payment, and others will only accept currency notes in denominations of $20 or smaller. Isn't this illegal?

The pertinent portion of law that applies to your question is the Coinage Act of 1965, specifically Section 31 U.S.C. 5103, entitled "Legal tender," which states: "United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues."

This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no Federal statute mandating that a private business, a person or an organization must accept currency or coins as for payment for goods and/or services. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise. For example, a bus line may prohibit payment of fares in pennies or dollar bills. In addition, movie theaters, convenience stores and gas stations may refuse to accept large denomination currency (usually notes above $20) as a matter of policy."