Heh, I probably made it a single pocket subconsciously to make things easier for myself while writing the demonstration 
When creating physical objects, you have to think about whether you want them to be single-named or plural-named, and what indefinite article you want them to have. I actually wrote a post about this in response to another question a week ago (Uncountable nouns - #2 by severedhand) ‘is/are’ on its own won’t make a difference, but the word before pockets will. ‘some pockets’ will create a plural-named objects, ‘a pocket’ will create a singular-named.
It’s usually best to think about the single/plural issue practically (for programming) rather than realistically. What I mean is, sure, a nightwear in real life probably has a couple of pockets. But do you want the player to actually have to specify which of two pockets they put things into? That would make for complex disambiguation, and annoyance for the player.
So the workaround here may be to create a single container in the nightwear, but we’ll call it ‘pockets’ and make it plural-named. And we’ll also let the player refer to it as ‘pocket’ (singular), even though behind-the-scenes it is plural-named. Here’s the demo of this stuff:
Lab is a room.
a carrot is in lab.
a nightwear is a kind of wearable thing.
the gown is a nightwear. player wears the gown.
some pockets are a kind of container.
pockets are a part of every nightwear.
Understand "pocket" as pockets.
Test me with "get carrot/put carrot in pocket/x pockets/get carrot".
So you could have two separate pockets, but this way is much easier for player and programmer.
Sorry, I don’t understand this question?
-Wade