Hi there!
Because it is a plugin parameter configuration:
As so, you need to put a script call, if you want, on that plugin parameter. Example in case you want to use a variable ID 10 to hold the max item slot:
- return $gameVariables.value(10)
Or just a number:
- return 15
The important thing is for it to start with "return ", no qoutes. And be whathever you put there, must result in being a number.