Torah Temimah cites Shevuos 38a.
“For one of all” — to make him liable for each and every one; five claimants… an oath “I have nothing of yours nor of yours” — liable for each one.
Ribbuy of multiple counts — liability for each distinct denial.
A single oath formula that denies several people or several types of claim multiplies the asham liability. Five claimants under one oath, or five categories of claim, each generate a separate asham obligation.