There’s no exact number, but according to a 2020 Chainalysis report, around 3.7 million of the then-mined 18.6 million BTC haven’t been moved in five or more years from their addresses. It’s safe to assume that most of them are burned. In other words, around 17% of BTC that will ever exist (21 million) is likely lost forever, and the percentage can still increase in the future.