These companies, backed by venture capital and worth $500 million to $999 million at a post-money valuation, can tell us a lot about the start-up world and where it’s heading, said Ilya A. Strebulaev, a Stanford professor with an expertise in finance and venture capital. There are now “dramatically more soonicorns in the United States than 10 years ago,” he added, noting that, by the end of last year, more than 2,000 companies had reached the status. Though Dr. Strebulaev said he did not invent the term, he has emerged as a prominent user.