Who will verify any transaction when the upper bound to total number of bitcoins is reached?
Please see here https://en.bitcoin.it/wiki/Transaction_fees .


  It is envisioned that over time the cumulative effect of collecting transaction fees will allow somebody creating new blocks to "earn" more bitcoins than will be mined from new bitcoins created by the new block itself. This is also an incentive to keep trying to create new blocks even if the value of the newly created block from the mining activity is zero in the far future.