I'd like to reset the counter of completed tasks. I can delete only one task at a time.
Unfortunately, that's not possible right now, but we will consider adding such option in the future. You're welcome to add your vote for this request in our Votebox: http://todoist.com/Vote/showProposal/409/
As an interim solution could you at least allow for the CRTL selection option in this view to be able to delete a large group of completed tasks at one time. This would greatly facilitate this sort of clean-up, but would reduce the likelihood of someone unintentionally deleting them.