There's a problem with your question. There's no such thing as a standard size scoop!
If you've not yet purchased a scoop, then you can buy one from here with a choice of sizes between 8 portions per litre and 50 portions per litre:
http://www.akroservices.co.uk/products.php?cat=393
(You don't need to be a mathematics genius to work out that the largest size will give you 40 scoops per 5 litre tub while the smallest size will give you 250 scoops per 5 litre tub).
If you've already got a scoop (and it's the same shape as those on the web page above) then you'll have to measure the diameter of the bowl to find the equivalent model and see how many scoops per litre you can expect to obtain. (Then, of course, just multiply by 5 to find the number of portions per tub).
Chris