[xConStruct]

Hmm, maybe checking against bag size, like:

local function getBagCount()
    for i=1, 4 do
        if GetContainerNumSlots(i) == 0 then
            return i-1
        end
    end
    return i
end

Just thinking if it is possible to have a free slot between two bags - then this method wouldn't work, but then you'd need to fetch their actual id's and not just the count.

Where do you need it?