The movie and novelization of Rogue One indicate that the destruction of both Jedha City and the Scarif complex required but a "single reactor ignition". So how many reactors does it take for the Death Star to fire a full blast?
(Actually, the "Explore" option on the Rogue One official website seems to indicate that there is only one reactor - called a "hypermatter reactor".)
So is there any explanation?
The implication seems to be not that the station has multiple reactors (all signs point to it having a single main reactor) but that they're referring to the amount of energy output from the hypermatter reactor itself.
Note that they use a series of baffles to reduce the amount of radiation, and hence power being generated by the main reactor. This presumably is what they mean by using a single reactor.
“Target Jedha City,” he snapped. “Prepare single reactor ignition.”
Krennic concealed his resentment, calmed himself with the sounds of his breath and the tidal rush of the station reactor. This wasn’t how he’d imagined the culmination of twenty years’ work—a diminished attack, a grand moff’s power play—but it was the reality he contended with.
...
KRENNIC PACED THE OVERBRIDGE UNDER Tarkin’s watch, observing the technicians and referencing every step against the control protocols he’d memorized long ago. Levers were flipped, rotating focusing lenses deep within the station’s core. Engineers adjusted radiation baffles and ventilation pumps as the main reactor shook with effort and its comforting roar turned to an eerie scream. Krennic saw more than one hand shaking, more than one face flushed or daubed with sweat. But his officers knew their duty. They would do everything necessary to destroy Jedha City at the bidding of their commander.
