For example, the very first block reward of 50 BTC can never be spent, as the coinbase transaction from the genesis block was never added to the transaction database.
The first block reward doesn't take part in the 21M rule as it doesn't take part in the first epoch. First halving occurred in block 210,000 and since the system halves the reward every 210,000 blocks, then genesis (AKA block 0) is excluded.
That's probably why Satoshi configured those coins unspendable.
It would be very interesting for someone to go through the entire UTXO set and total up the number of provably lost coins which could be subtracted from the maximum possible supply.
I'm not sure if coinmarketcap.com has had the correct supply: 18,836,593 BTC (as of block 703,865)
210000*50 + 210000*25 + 210000*12.5 + 73,865*6.25 = 18,836,656.25 BTC.
These 63.25 BTC aren't counted and are, most probably, provably burnt.