You will want to go to the settings for the relationship between 'tasks' and 'sub-tasks'.
In there you can click the button to 'add summary'
Don't change any of the options when it loads. By default it will count all sub-tasks associated with the parent task.
Click save, it will probably prompt you to name it "# of Sub-tasks". Change the name if you'd like.
Save.
That field be default will also be a link to all the subtask records, but will also be a count.
Cheers,