system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Browsing this sub i came to know that the emperor is kept alive with sacrifices.

Are they volunteers who are sacrificed to keep the emperor alive or criminals. And how are able to sacrifice millions every day without a rebellions. And is it worth it to keep him alive?
gpt
Thats... not exactly true. 

First off, it's not millions every day. It's an average of 1000. The number's actually gone up as time has gone on, but it's actually pretty low for 40k.

Second, they aren't just regular people, but Psykers, collected from across the Imperium, by the Black Ships. While we aren't entirely sure, it's believed that these are still operated by the Sisters of Silence, a order of Psychic Nulls who have served as the Emperor's Psyker-hunters since the Great Crusade. 

Third, they aren't actually sacrificed to keep the Emperor alive, but rather used as living batteries to power the Astronomican. Over a period of weeks or months, the individual Psykers wired into the beacons Psychic Choir have their souls burned away and need to be replaced.

The only Psykers that actually get brought before the Emperor are those of strong enough will to control their powers. They undergo the Soulbinding, which almost always blinds them, and serve as the Astropath's and Sanctioned Psykers of the Imperium. 

As for whether or not it's worth the effort... Without the Astronomican, the Imperium literally loses it's primary means of navigating through the Warp. Warp travel will still be possible, but knowing where you're going will become far more difficult, and would probably lead to the total destruction of the Imperium.