Tech CEOs seem to have decided that when it comes to layoffs, about 6% of the workforce is a magic number. As today’s job report showed, the U.S. economy as a whole was still humming along nicely in January. But in the tech industry, the new year got off to a depressing start, with Amazon announcing on January 4 that it would be laying off 18,000 employees, or 6% of its corporate workforce.