That.
For instance ... The man said that that 'that' that that other man used was wrong.
That's five.
The man said that that 'that' that that 'that' modified was wrong.
That's six.
Did the man know that, that that 'that' that that 'that' followed was redundant?
And that's seven.
They're not an elegant use of English, but they're correct. And that's that.