Seven “thats” may follow each other, and make sense.
For be it known that we may safely write Or say that `that that' that that man wrote was right; Nay, e'en that that that, that `that THAT' has followed, Through six repeats, the grammar's rule has hallowed; And that that that that that `that THAT' began Repeated seven times is right, deny't who can.
“My lords, with humble submission that that I say is this: That that that `that that' that that gentleman has advanced is not that that he should have proved to your lordships.” —Spectator, No. 86.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894