>>96407798
Ignore autistic lore in the novels/new books.

The first blackbook is very clear, see pic related.The way everything else is organised makes sense if its true (i.e. a centurion, a company commander is in charge of 100 dudes, hence the name, unlike nulore where a company is 5 trillion marines).

Of course the blackbook also is clear that legions began to vary away from these original numbers, but it says this tended to be at the chapter level. i.e. some legion's chapters (or chapter equivilents) could be a lot more than 1000, but generally a company is still 100 dudes.