Judging by the first weekend's reports, "Conan the Barbarian" packs a lot of box-office muscle: The sword and sorcery saga with Arnold Schwarzenegger as Edgar Rice Burroughs' warrior hero made an estimated $10.3 million in its first three days of release despite a batch of decidedly mixed reviews. But if "Conan" holds up at the turnstiles and becomes the big summer hit that many people expect, how will there be room for all the other big summer hits waiting in the wings? After all, somebody has to be left to see "Star Trek II: The Wrath of Khan" and "Poltergeist" and "The Extra Terrestrial" and "Annie" and "Rocky III" and "Grease 2" and "Tron" and "The Best Little Whorehouse in Texas." And also the scheduled re-releases of the old blockbusters "Raiders of the Lost Ark," "Stripes," "Superman II" and "Bambi." Every one of those either has already been a hit or has a lot of people expecting it to be a huge success, but there are also bound to be a few monumental flops in the lineup. The inside line these days is to bet on "Whorehouse" to fail, simply because those who've seen it say it's awful; "Annie" is another question mark, since it appeals to the young children and older adults who normally don't make up a good moviegoing audience. But the advance word is extremely positive on a couple of the space films: "Star Trek II" (in which, yes, Mr. Spock does die, but he might be back anyway--just ask Obi-Wan Kenobi, who will be resurrected for next summer's "Revenge of the Jedi") and Spielberg's "The Extra Terrestrial," formerly "A Boy's Life," formerly "E.T. and Me," formerly "E.T." and reportedly the only movie ever made about an alien that's so touching it will reduce an audience to tears.