How many living children do the Capulets have?
Juliet is their only living child at the time of the play. Lord Capulet tells Paris, "She is the hopeful lady of my earth" (I, ii), meaning that Juliet is his "heir and hope for posterity" (David Bevington's The Necessary Shakespeare). Because Capulet calls Juliet, a daughter, his heir we must assume that he has no son to inherit or continue the family name. In addition, no other child is mentioned - no siblings to Juliet - so I think it's a safe bet that they have only the one living child.