The dominant online video company said on Tuesday it had 74.8 million subscribers at the end of December and forecast 6.1 million more through March, fueled by its expansion this month into virtually every country except China, where it is exploring ways to launch its service. Shares of Netflix rose 7 percent to $115.42 in after-hours trading. New customers overseas are countering slowing growth for Netflix in the United States, the company’s biggest market.