If you added a user set for recipients, the number of Recipients will appear against your mailshot. If you did not, then it will show as 0. If you selected a user set, you cannot remove it but you can edit it.
If you have not added any recipients, or to edit the existing user set, click on the Recipients icon. This will take you to the User sets area where you can create a User set for your recipient list or edit the User set that you selected earlier.
When you save the User set, you will be taken back to your list of Mailshots and the number of people in your User set will appear in the Recipients column.