trillions

Sa'cea was not an easy sept to establish. Its worlds are hot, lashed by ferocious natural phenomena and the fiery anger of their star. The T'au not only overcame these challenges, but they also established such numerous and prosperous dome cities that Sa'cea Sept is now the most populous of all T'au controlled systems.
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The worlds of Sa'cea Sept boast interconnected dome cities and agriplexes that cover almost the entire surface. Beneath their foundations, subterranean cavern networks contain sprawling complexes of Earth Caste laboratories and high-tech factories that labour in a continuous artificial day. Sa'cea Prime is a sweltering cityscape of glittering crystal and alloy whose once-inhospitable deserts now play host to teeming trillions.
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Human worlds supporting such vast populations have been shown time and time again to languish in inequality and near anarchy; the T'au look upon the nightmarish hive cities of the Imperium with mingled pity and revulsion. Thanks to the strictures of the Greater Good, the cities of Sa'cea are nothing like this. It is true that the average citizen lives a comparatively spartan life when set against T'au of other septs, yet every Sa'cean does their part to ensure that life in their cities remains manageable, their worlds productive and their people healthy. This requires no little individual discipline and selflessness. Not for nothing are the T'au of this sept renowned for their calm temperament and unimpeachable conduct.

Codex: T'au 9th Edition