What does Ellen want to be when she grows up in Number the Stars?

When Ellen grows up, she would like to be an actress.

Ellen is "a talented performer; she often play(s) the leading roles in school dramatics".  She has a vivid imagination and the ability to make games of make-believe fun.  When Mrs. Johansen had told Annemarie and Ellen "the entire story of Gone With the Wind in the better days before the Nazis began their oppression in Denmark, the girls were enthralled by the "interesting and romantic" tale, and enjoyed acting out the parts in it for fun.  While Annemarie without question likes to playact for fun, it is Ellen who has a real gift for dramatics; Annemarie herself tends to be more athletic, while Ellen's first love is the stage (Chapter 4).

Mr. Rosen wants Ellen to be a teacher when she grows up.  He himself is one, and Ellen complains that "he wants everyone to be a teacher, like him".  Ellen herself would like to go to acting school, and hopes to be able to convince her father to allow her to pursue her dreams when the time comes.  Annemarie, recognizing Ellen's dramatic talents, encourages her to pursue her interests, telling her, "You were great as the Dark Queen in the school play last year...you should be an actress when you grow up" (Chapter 5).