Instance member cannot be used on type

You just have syntax error when saying = {return self.someValue}. The = isn’t needed.

Use :

var numPages: Int {
    get{
        return categoriesPerPage.count
    }

}

if you want get only you can write

var numPages: Int {
     return categoriesPerPage.count
}

with the first way you can also add observers as set willSet & didSet

var numPages: Int {
    get{
        return categoriesPerPage.count
    }
    set(v){
       self.categoriesPerPage = v
    }
}

allowing to use = operator as a setter

myObject.numPages = 5