question

The maximum can't always be calculated. You can have no maximum, where items can always successfully acquire a slot. You can also have the maximum depend on the items that appear. For example, supposed you limit slots by their available volume, and you put randomly-sized items in the rack. You could probably calculate a range for the maximum in that case. There are many other situations that make calculating a true maximum impossible.

If you want to count slots, here are two ways to do it:

Storage.system.querySlots("").length

or

int count = 0;
var objects = Storage.system.storageObjects;
for (int o = 1; o < objects.length; o++) {
    var bays = objects[0].bays;
    for (int b = 1; b <= bays.length; b++) {
        var levels = bays[b].levels;
        for (int v = 1; v <= levels.length; v++) {
            var slots = levels[v].slots;
            for (int s = 1; s <= slots.length; s++) {
                var slot = slots[s];
                if (slot.isStorable) {
                    // You may need to increment the count based
                    // on slot labels or something
                    count++;
                }
            }
        }
    }
}
// for a script window:
// return count;