The New York University professor who developed one of the best-known formulas for predicting corporate bankruptcies has a warning for U.S. credit investors: this year’s spate of “mega” insolvencies is just getting started. More than 30 American companies with liabilities exceeding $1 billion have already filed for Chapter 11 since the start of January, and that number is likely to top 60 by year-end after companies piled on debt during the pandemic, according to Edward Altman, creator of the Z-score and professor emeritus at NYU’s Stern School of Business. Global firms have sold a record $2.1 trillion of bonds this year, with nearly half coming from U.S. issuers, data compiled by Bloomberg show.