The number of officially reported AIDS cases around the world has already risen by nearly 7% this year, the World Health Organization said in Geneva. By the end of February, 145 nations reported a total of 141,894 cases--8,918 more than at the end of 1988. WHO, a U.N. agency, believes that in reality 300,000 to 350,000 have AIDS and that not all cases are reported. It believes that 5 million to 10 million people carry the AIDS virus without having developed the disease. One million will contract the disease by 1991, it says. The United States reported the most cases--86,157. The figure was 80,538 at the end of 1988.