How many people would get killed if an atomic bomb struck New York?

No answer can be given to this without knowing much more information as there is no "standard" atomic bomb. First you need to know the yield (i.e. explosive force) and atomic bombs have been designed with yields ranging from as small as 10 tons to 100 million tons of TNT equivalent. Second you need to know the altitude at which the atomic bomb exploded, as the higher the altitude the more the blast can spread to do damage (but above the optimal altitude for that yield the blast weakens too much as it spreads). Third you need to know how close to the center of population density ground zero was, as if this is missed by a wide margin (as it was at Nagasaki) damage and fatalities are significantly reduced. Given these three factors and the necessary effects equations a reasonable estimate for the answer to your question can be made.

However many other factors also affect the damage and fatalities caused by an atomic bomb, a few are: terrain, building types, weather, if the bomb was a clean or dirty design, etc. To calculate a correct answer for the effects of a given atomic bomb detonation all of these must also be included.