system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] We know there are untold billions of humans, but how many are there of the other species?

An easy place to start might be, how many Adeptus Astartes (still technically humans, but only technically) are there across all of the chapters?

After that, are there any ways to estimate the Eldar, Tau, or Necron populations? It obviously becomes much harder with species like the Orks or Tyranids, but is there any way to estimate those populations?

*alternate question to be able to get slightly more tangible answers: For those beings who inhabit planets, what would the population for an Earth-like planet be. How many humans to a hive world? How many Eldar to a Craftworld? How many Necrons would normally be entombed on a single planet? Etc, etc.*
gpt
>how many Adeptus Astartes (still technically humans, but only technically) are there across all of the chapters?

1 Million, roughly.  A thousand Chapters is described in the codices.

>After that, are there any ways to estimate the Eldar

No.  You've got Harlequins hanging out in the deep Webway, the Dark Eldar in the less deep webway, unknown number of Exodite worlds, unknown number of craftworlds total.

>Tau

See above.  Unknown number of total worlds, do you want Tau only or down to the castes or including the auxiliaries?

>Necron 

See above - unknown number of tomb worlds in existence, never stated how many Necrontyr were given metal life.

>Orks or Tyranids

For all intents and purposes, infinite.