The Megilah tells us about the episode of Esther being taken to palace of King Achashverosh together with all of the other girls. Hagai, a minister of the king, noted Esther’s potential in becoming the next queen. He rushed to arrange a meeting with the king and also put at her disposal שבע הנערות seven maidservants.
The Gemara in megillah (13,A) tells us that Esther used these maidservants to count (keep track of) the days of the week. Did Esther really need seven maidservants, one for each day just to know when Shabbos would fall out? We know that Esther was one the seven prophetesses that served the Jewish nation, did she really need the seven maidservants to know when Shabbos falls out?
R’ Yonason Eibshits in his sefer Yar’os Dvash sheds light on the matter. He explains that Esther was concerned of her true identity being revealed in the palace. In order for no one to be suspicious of her behavior as a Jew, not even her close aides, she used a different maid every day. Each maid assumed that Esther acted the same way every day. Therefore the maid designated for Shabbos did not suspect that Esther was acting different that day; she assumed that Esther acted in that peculiar way each day of the week. Through this Esther kept her identity a complete secret. The weekday maidservants assumed that Esther does ‘melacha’ every day of the week including on the Shabbos, and the maidservant of Shabbos assumed that Esther has weird customs of not performing certain activities and she lives by that each day of the week.
Based on this we can explain another point. When Achashverosh wishes to summon Vashti to his party the pasuk lists the names of the seven closest advisers to Achashverosh and finishes by saying, "שבעת הסריסים המשרתים את פני המלך אחשורש". Is it not redundant to mention the advisors by name and still add seven servants? We can all count for ourselves, why mention the number? A few psukim later when Vashti refuses to appear at the party, the pasuk mentions Achashveroshs’ seven advisors by name and again finishes by mentioning the number seven despite their count being obvious "שבעת שרי פרס ומדי".
This can all be explained based on what has been clarified above. Since Esther needed seven maidservants to properly conceal her national origin and Jewish identity from the people in the palace, Hashem already prearranged this arrangment as the normal custom of the royal family in Shushan. The king was aided by Seven servants in all matters; seven maid servants, seven advisors and so on. This being the case, it was only natural for Esther to also receive seven maidservants, for she was to be the queen and just as the king had groups of seven for the services he required, so too the queen had seven maidservants, therefore no suspicion was aroused.
This also explains why the pasuk adds "הראויות לתת לה מבית המלך", since the custom was groups of seven servants Esther didn’t even need to ask for this, instead she immediately received what she needed- the seven maidservants.