SAN JOSE, Calif., Nov. 15 — Each fall for more than two decades, the high-tech world has descended upon Las Vegas to hype the industry’s hottest wares and ideas at the Comdex trade show. Whether focused on the first PCs of the ’80s, the Internet in the ’90s or the wireless gadgetry of recent years, exhibitors were assured hordes of executives, buyers and reporters. But at the 23rd annual show, which starts Monday, the crowds will be smaller, the exhibit area more compact and the mood subdued. Comdex, like the rest of the high-tech industry, is sputtering.