Board Thread:Questions and Answers/@comment-30488984-20180630192929/@comment-24909831-20180701113548

"On paper, each of the Chapter's eight brotherhoods contains roughly one hundred battle-brothers under arms. [...] does not include the brotherhood's officers [...]"

- Codex: Adeptus Astartes - Grey Knights (8th edition)

I don't know where they've got the "1800 marines" in 40k Wikia, but as I have said before and say now again; 40k Wikia is full of fanfic, fan speculation and uncited sources, read it with shitload of salt.

By my calculations, based upon the 8th edition codex page 13, there are: After the return of the Planet of the Sorcerers. Then there's some amount of servitors and equerries.
 * 1062 battle-brothers and officers, including tech-marines, apothecaries, librarians, paladins, purifiers and Grand Masters, ancients and captains
 * 19 Dreadnoughts
 * 12 Venerable Dreadnoughts
 * 32 Neophytes
 * 1005 recruits