The Academy of Television Arts and Sciences said Wednesday that it plans to expand the number of nominees in several categories from five to six. It indicated that it was doing so because frequently the sixth-placed finisher received nearly as many votes as the fifth placed. The Academy Is also changing the method for selecting the nominees. Previously, the entire membership would vote in each of the categories and the top contenders would then be appraised by a blue-ribbon panel, which would make the final choices.