I thought they'd be able to squeeze at least 2 seasons given how content rich the book is.
‘SHOGUN’ will likely not get a Season 2. “We love how the book ends; it was one of the reasons why we both knew we wanted to do it — and we ended in exactly that place.” (Source: https://hollywoodreporter.com/tv/tv-features/shogun-feudal-japan-season-2-chances-showrunners-interview-1235845407/)