only 21 million bitcoins to be mined okay ? what will happen when miner had mined 21 million bitcoins or there is nothing like my 21 million bitcoins to mined theory?
In about 100 years when all Bitcoins have been mined, the miners will mine for the Transaction fees, as I already told you.
This past week Transaction fees have approached the amount of the block reward. 100 years in the future the fees will only be a higher amount.
I have answered your question multiple time, ASAP!! So you don't need to worry about for 90 to 100 years. Study how Bitcoin works and these very basic questions get answered. This is explained in the bitcoin.pdf that I gave you a link for.