You misheared... There is no limit how many blocks will ever be mined. However, the block reward will halve every ~4 years, so in the end there will be just under 21.000.000 BTC in circulation. (altough, many of those unspent outputs are funding addresses whose private key belong to satoshi, or to people that have lost them, so eventough there are potentially 21 mln BTC in circulation, only a small fraction of those is actually "spendable"). Miners can still mine after the block reward has dropped to 0, they can mine to collect the sum of the fees of all transactions they include in the block they're trying to solve.