E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.

That.

Seven “thats” may follow each other, and make sense. 1

“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.