I need to store the amount of objects in scene in a var. How?

I have all of these objects in a scene under one tag name, Pickup. I want to have a script that finds all of the objects in the scene under that tag name and store them in a variable for later use. I could just write them in the script but I want a script that I can reuse in other levels.

Just get the objects with GameObject.FindGameObjectsWithTag and store its length in your variable - like this:

var pickupAmount: int; // declare the variable outside any function!

function Start(){
    pickupAmount = GameObject.FindGameObjectsWithTag("Pickup").Length;