How many Bitcoins
are left?

Bitcoin becomes harder to issue over time.

New bitcoin enters circulation through the block subsidy. Roughly every 210,000 blocks, the subsidy is cut in half. That programmed schedule causes issuance to slow progressively while the maximum remains fixed.

“Left to be mined” describes protocol issuance, not necessarily liquid bitcoin available for sale. Coins can be lost, held long term or otherwise unavailable to markets.