So, what you are asking is not that will there be more then 21M "real" BTC but how would we know if bank A says they have 1M BTC and bank B says they have 1MBTC and so on other then a complete audit of everything they have how can we know that they really have it?
If that is what you are looking for then the answer is well...we will not without them providing a full audit.
Financial institutions are supposed to be audited by professionals who have their work supervised and checked by others. But, as we saw in 2008 and 2001 and all through the 1980s and early 1990s and probably before that people lie.
That is what is great about BTC You can hold your own.
Not your keys....not your coins....
-Dave
If it's regulated by the SEC, this all has to be disclosed. ETFs are, and it's fully disclosed as required by law. That can't be hidden. HOWEVER..the new fund in town is the CIT (collateral investment trust) or also known as CIF (collective investment fund). These funds have now surpassed Mutual Funds as the top fund type inside 401k's. Currently they can only be purchased through employer sponsored retirement plans (401k's, 457'bs and look to be available in 403b's very soon).
I have a real issue with them, and work with them daily, so I know them well enough to know I don't feel warm and fuzzy about them. They are NOT overseen by the SEC, which means less regulation and less transparency required. Their Morninstar Whitepapers look very different than those of Mutual Funds and ETFs. I know the plan I work with, the CIT's are managed by the State Comptroller.
Here's more info on them -
https://www.investopedia.com/terms/c/collective-investment-fund.asp