The headline figure is $500,000 for each customer, with a sub-limit of $250,000 for cash awaiting reinvestment. The limit applies per separate capacity, not per account number, so an individual account and a joint account are counted separately while two individual accounts at the same firm are combined.
Securities are returned in kind where possible. If 90% of the missing shares are recovered from the estate, SIPC funds the rest up to the limit, valued as of the filing date. That valuation date matters: a volatile stock can move a long way between failure and payout.
Coverage does not extend to futures positions unless held in a securities account under a portfolio margin arrangement, nor to currency, commodities, or most unregistered investments. Some firms buy excess-sipc-insurance on top.
Related: sipc, excess-sipc-insurance