How many "Sequels" is resonable figure?

Though no follower of Sharpe, I read quite a bit of Bernard Cornwell's other stuff.
This lot is a bit of a dramatisation of the American Uncivil War.
He says ..." All the battles and Skirmishes in the novel are based on real actions ......." He, even includes some of the actual, participating, Officers.
However ...
"Copperhead" is listed as "Sequel to the No 1 bestseller Rebel" and
"Battle Flag" as "Sequel to the bestselling Copperhead".
Following these comes ..."The Bloody Ground"
NOT Listed as a sequel to a sequel to a sequel. I wonder why?