I don't know exactly how many missions there are.

Developers say the number of missions is 18. However, the game community claims that the number of missions is 13. Then I don't know what the remaining five missions mean. I don't understand why the words are different.

I don't think the developers ever mentioned 18 levels. They might have said it was an 18 to 20 hour-campaign, though. The game is already in the hands of a few people, and they can confirm that there are 13 levels in the game.

1 hour ago, Walker said:

Developers say the number of missions is 18. However, the game community claims that the number of missions is 13. Then I don't know what the remaining five missions mean. I don't understand why the words are different.

https://store.steampowered.com/app/782330/DOOM_Eternal/

What is DOOM Eternal?
DOOM Eternal is a fast-paced, violent, first-person shooter pitting the player against hordes of demonic enemies. Users can fight through 18 missions of intense single-player campaign, or face off against 2 demon players in the asymmetrical multiplayer arena: BATTLEMODE.

Please look at this. Isn't this FAQ clearly referring to 18 missions?

Huh, then I don't know either. It being a simple typo seems plausible, but unlikely. Maybe that number includes the "Master Level" versions of some maps?

There are 13 levels, plus the Fortress of DOOOOOOOOOOOM!!! hub level and its "ripatorium" [sic].

But what's a mission? Is it a level, or is it a goal? Are the "slayer's gates" separate missions? Or, yes, the "Master Levels"? If we imagine a few levels have more than one "mission" in them, then you can perfectly well get to 18 missions in 13+1 maps.

Or we can imagine a future DLC bringing in five more maps, and they took this into account to get to 18. I dunno.

There is a thread right below that gives us the final level count.  It has the names of them and might be a little bit of a spoiler.  FYI there are 13.

There's 13, same as last game.

Maybe they accidentally counted the master levels or something else when they said 18.