The Maharal writes that Esther’s servants fasted because, if Achashverosh accuses Esther of lacking fear for him by approaching him without being summoned, she can explain that she feared him so much, she fasted. If she is no believed, she can point to the fact that even her servants fasted.
According to the Ohel Moshe, Esther has her maidens fast because of the Talmudic concept (Brachos 34b) that the agent of a person is the extension of that person.
The Sefer HaChaim writes that, grammatically, atzum (“I will fast”) is in the singular, and Esther meant that she could only encourage her servants to fast.
The Maharal writes that the idea that there is power in numbers is only relevant for men because women never lose their individual identities, as men should in gatherings like a minyan. This is why Esther uses the singular form of the word here.