Of all the Mask monsters in Arkham Horror, there are 5 in the base game, 1 in Dunwich Horror, 2 in Kingsport Horror, and 2 in Insmouth Horror. If Nyarlathotep is the Ancient One, do I add all Mask monsters from the base game and any expansions I am playing to the cup? Or do I cap them at a certain number (say, pick 5 random Mask monsters)?
You use them all.
The closest the rules get to directly saying this is things like this from the Dunwich Horror rules:
As with the “Mask” monster markers from Arkham Horror, do not place the Spawn monsters in the monster cup (unless the Ancient One sheet instructs otherwise).
That strongly suggests that you're expected to use them along with the original ones if the Ancient One says to. The Mask monsters are also referenced a few other times in the rules, and similarly in the Innsmouth Horror rules, every time just implying they're treated the same as the original ones. If they wanted you to randomly select 5, I think they'd have to spell that out, the way they very clearly did with Ally selection.
Also, this makes sense. The idea of having 5 masks in the base game was to give them a certain probability of being drawn. The expansions all add additional monsters to the cup, and if they didn't also add Masks then the odds of drawing a Mask would go down.