Now that the studios are so delighted over the summer prospects, they're already preparing to go all out and make 1982 a spectacular movie year--by gearing up for the biggest Christmas season in some time. And it will unquestionably be the biggest if nothing but quantity is measured: The larger studios are lining up openings for 20 major Christmas films, a one-third increase over last winter's already large total (although so many of the films released last December were gritty, realistic dramas that few of them made substantial holiday money and lighter entries like Disney's "Cinderella" reissue became unexpected blockbusters). Scheduled releases include the Dudley Moore/Mary Tyler Moore drama "Six Weeks"; Meryl Streep in "Sophie's Choice"; the Linda Ronstadt version of "The Pirates of Penzance"; new Burt Reynolds and Clint Eastwood films ("Best Friends" and "Honky Tonk," respectively); the Jackie Gleason/Richard Pryor comedy, "The Toy"; director Sidney Lumet's "The Verdict," with Paul Newman; "King of Comedy," a Martin Scorsese film starring Robert De Niro and Jerry Lewis; "Airplane II: The Sequel"; and "Tootsie," a film that currently has Dustin Hoffman running around the streets of New York City dressed in women's clothing.