I mean its only 21million btcand there are 6billion people in the world , how can it be enough
It's not really about the quantity of coins there is.
Think about it, there are countries where exchange rates are really low, say South Korea and Japan. And there are countries where there is a relatively low amount of money in circulation and a really high exchange rate as an result, let's just say Great Britain and Europe.
21 million BTC can be divided into infinite sub-units, it can be seen as 21*1000 million mBTC, or 21*1000000 uBTC, or 21*100,000,000 satoshis. Now is that enough for the whole world? Yes.
It doesn't matter how many bitcoins are in circulation. What matters is if the network is functional or not.