> It can be difficult to figure out whether the theoretical limit is 10x or 100x in my mind because there isn't a reserve ratio federally (well, there is one, but it's zero)

I know what you're thinking of here, but it doesn't mean anything like what you think it means.

So the US used to have a rule that every bank hand to have a certain percentage of its assets stored in its account at a Federal Reserve bank; it is this percentage which was gradually reduced to 0 by I think 2020. Note that only the funds in that account meet the requirement; a literal pile of cash contributes not a single cent.

The way banks are primarily limited nowadays is via capital adequacy ratio, which is essentially that you need to set aside a particular pile of capital that can be raided to guard against assets falling in value to 0. It's complicated because this pile of capital doesn't come from the money a customer deposits in their account (which needs to be held as an asset to offset the liability a depositor represents), but rather from income the bank makes in other ways. If a bank sells $1 million worth of shares, they get to issue ~$20 million more loans.

If a bank gets $1 million worth of new deposits, they get to issue... $0 more loans. Well, maybe less: if a bank gets $1 million worth of new bitcoin deposits, that probably reduces its capital ratio because bitcoin is such a risky asset.