Assuming you actually have a size account and this isn't total bullshit here's how it works.
First securities held by the firm are identified - if possible - and then returned. Then SIPC coverage is $500,000 with no more $250,000 in cash. Supplemental insurance is somewhat deceptive. Mostly underwritten by Lloyds with an aggregate cap. The aggregate cap is the issue you should concern yourself with. Then firm capital, but there would be none left in a failure. Market cap of the stock is pretty much irrelevant as that is going to be vastly reduced and some/or most of it is outside of the firm. So Chairman of Lehman's personal net worth out of Lehman isn't part of the equation. His Lehman stock went worthless, but you have no claim against his other assets except with a proof of fraud. Any REAL size account would hold the assets at an outside trust company. Which are not insured if fraud or failure - look at Intrust in Chicago which failed and left lots of account SOL. You would use DTCC, JPM or a few of the others and pay for custody. Remember most of this stuff is electronic book entry and most size accounts are really looking to earn as much as possible through stock loan rebates and would never leave the holdings just sitting in a brokerage account.