STANFORD, Calif. — Top medical experts studying the spread of Ebola say the public should expect more cases to emerge in the United States by year's end as infected people arrive here from West Africa, including American doctors and nurses returning from the hot zone and people fleeing from the deadly disease. But how many cases? No one knows for sure how many infections will emerge in the U.S. or anywhere else, but scientists have made educated guesses based on data models that weigh hundreds of variables, including daily new infections in West Africa, airline traffic worldwide and transmission possibilities.