Portfolio heat is the total risk across all open positions at once, not just the risk on any one trade.
Six unrelated positions each risking 1 percent put 6 percent of the account at risk together, and correlated positions are far more likely to all hit their stops in a single move, so the full 6 percent can be lost at once. Prudent traders cap total open risk (commonly 3 to 6 percent) on top of the per-trade limit.
Formula / example: 8 correlated 1% trades can lose 8% in one move