is there a limit on how many bitcoins can be mined? on how many bitcoins will circulate the internet world?
That's what this chart is showing. The blue line is the number of bitcoins in existence as of the block number shown on the bottom axis. The number of bitcoins is exactly a function of the number of blocks, and the function is asymptotic. The years shown on the top axis are approximate, based on the target block rate of one block per 10 minutes. In reality, we're somewhat ahead of schedule because the hash rate has been increasing so aggressively.