Jesus knew the number of demons in a person.
          Mark 1:26 casted out one -   And when the unclean spirit had torn him, and cried with a loud voice, he came out of him.
               Mark 1:32 casted out many -  And at even, when the sun did set, they brought unto him all that were diseased, and them that were possessed with devils.
          Mark 5:9 casted out legions - And he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.