how can I use add_action with external class which the function contain 2 argument?

The publish_post post takes two arguments first is $ID of the post that is being published and the second is the $post instance (See codex). I modified your code a bit below.

Your class is almost identical just renamed the parameters of the function to not cause confusion.

class customClass{ 
   function dothisfunction( $ID, $post ){
      echo "something";
   }
 }

No need for the $number and $mymsg in the array() because you only need to specify instance and the method to be used of the instance.

$myClass = new customClass();
add_action( 'publish_post', array( $myClass ,'dothisfunction' ), 10, 2 );

Also, you should specify how much parameters you are passing to dothisfunction in add_action as 4th parameter.