Re: What would you pay for the keys to the genesis block/address

so is the total number of "usable" bitcoins, assuming none are lost to accidents, 20,999,950 instead of 21 million?

This is a thorny topic and really depends on what you mean by "usable" and "lost to accidents". Of the oft-cited 21 million BTC total:

some are fully spendable today.
some have yet to be generated (do not yet exist).
some are "lost" because a corresponding private key was lost (8999 BTC example by Stone Man) but are spendable in principle (cryptography breakthrough, quantum computers). Some of these will be easier to find than others: Casascius coins for example made us of the mini private key format and so are protected by 128 bits of entropy rather than the usual 160 bits.
some are at addresses for which it is not known that a corresponding private key exists (1111111111111111111114oLvT2; The Bitcoin Eater address; cryptograffiti.info/)
some are unspendable due to protocol bugs (the 50 BTC in question and the 100 BTC mentioned by zebedee above)
some are locked by scripts which provably return false on any input. Some of these are so badly lost they don't even have addresses! (367.75849319 BTC example).
23.1 mills (23.1 millibitcoins = 0.0231 BTC) may well never be mined into existence in the first place due to rounding all subsidies down to the nearest satoshi.
some have already lost the chance to ever be mined into existence (example by midnightmagic: block #124724 mints only 49.999 999 99 BTC).

Note: I use "exist" here for any amounts of BTC in an up-to-date unpruned Bitcoin UTXO database. There are of course different interpretations of what it means for a bitcoin to exist. Even this seemingly simple definition is not water-tight as two different full nodes may disagree on the UTXO and in particular it allows one to claim that at one point, there existed more than 180 billion bitcoins.

And this is why I said ASSUMING NONE ARE LOST. From the very beginning, there was never a possibility of having 21,000,000 usable bitcoins, or, is there an extra block to account for the unusable first block, and the amount from the first block is usually left out of the total?

Quite a simple question, and while I appreciate the two attempts to answer my question, and it seems you guys put a lot of time and effort into explaining, my question was not really answered.

Thanks for the compliment. I do pride myself on trying to actually answer questions here and not second-guess people.

I did read the question and took "assuming none are lost" on board. Unfortunately, although this question seems simple, it is actually rather muddy and depends acutely on what is meant by "lost".

I can confirm that there is no "extra block" to make up for this 50 BTC output. The fifth point of my cursory categorisation above claims this 50 BTC output to be part of the 21 million total.

Even before genesis, Satoshi did not intend for 21 million usable bitcoins to even be theoretically achievable. The subsidy rounding code, for the sake of simplicity, sacrifices 23.1 mills (23.1 millibitcoins = 0.0231 BTC), leaving a theoretical maximum valid UTXO total of 20 999 999.976 900 00 BTC. (Aside: At one point the intention was for a maximum of just 20 million BTC).