The Finch household usually consisted of four people, but when Aunt Alexandra came there were five.
Atticus’s wife died when Scout was two years old. As a result, Scout “never felt her absence,” but Atticus and Jem did miss her. Atticus never remarried, and instead raised his two children as single father. He did have help though, in the form of Calpurnia.
Calpurnia was the fourth member of the Finch household. She was an African-American housekeeper who served as a surrogate mother to the Finch children. Atticus relied on her completely. Atticus explains to Aunt Alexandra that his children never suffered from being brought up by Cal.
If anything, she's been harder on them in some ways than a mother would have been... she's never let them get away with anything, she's never indulged them the way most colored nurses do. She tried to bring them up according to her lights, and Cal's lights are pretty good- and another thing, the children love her." (ch 14)
When Aunt Alexandra comes, she upsets the balance of the household some. Besides wanting Calpurnia to leave, she also does not agree with how Atticus raises Jem and Scout. She wants them to be raised to know their place in society as “Fine Folk,” when Atticus just wants them to learn to be good people.