牌兒為號 A Tally For A Sign
小妖道：「也罷；我們這巡山的，一班有四十名，十班共四百名，各自年貌，各自名色。大王怕我們亂了班次，不好點卯，一家與我們一個牌兒為號。你可有牌兒？」小妖道：「也罢；我们这巡山的，一班有四十名，十班共四百名，各自年貌，各自名色。大王怕我们乱了班次，不好点卯，一家与我们一个牌儿为号。你可有牌儿？」
逐句解 Line by line
The little demon said, "Very well;
也罷 is a grudging all right then — the concession of somebody who has decided to check rather than argue.
we mountain patrols run forty to a shift and four hundred over ten shifts, each with his own age and looks, each with his own name and description.
一班有四十名 and 十班共四百名 — the establishment set out in figures again, and the figures multiply cleanly. 各自年貌 and 各自名色 are the two columns of a personnel record: age and appearance, name and category.
The king was afraid we should get our shifts muddled and be hard to call over, so he gave every one of us a tally for a sign. Have you got a tally?"
亂了班次 is to get the shift order confused; 點卯 is the roll call Bajie joked about, back when the number was a joke. 為 is wéi, to serve as. The demons have solved a staffing problem with identification badges, which is the most human thing anybody does in this chapter.
The little demon's answer to a suspicious stranger is not to attack him or raise the alarm. It is to ask for identification.
And the reason he can is set out first, in numbers that work. Forty to a shift, ten shifts, four hundred men, each with a record of his age, his face, his name and his grade. This is a personnel system, and it exists because four hundred lookalikes in the dark are hard to call over.
The chapter has been building this joke since the kings of hell were made to sign. The terrifying thing about the Lion Camel Ridge is not that it eats people. It is that it has a filing system.