When Doctor Strange confronts the Ancient One, she deflects by pointing out that the zealots will soon return and they will need reinforcements. She leaves, presumably to gather said reinforcements. But when the zealots return, she intervenes by herself, with unfortunate consequences. What happened to the reinforcements she was supposed to have summoned? Why didn't she bring them along?
More logically, you must remember that the Ancient One is drawing from the Dark Dimension, just like Kaecilius and his followers. Mordo points out that a connection with the Dark Dimension gives one more power in the mirror dimension, thus why Mordo and Strange are running from Kaecilius and his minions instead of fighting against such overwhelmingly powerful foes. The only reason the Ancient One is able to hold her own against them in that fight is because she also has a connection to the Dark Dimension. So even if she had brought reinforcements, they would have been as powerless as Strange and Mordo, and the fight would have been a slaughter.