The first question is fine. Perhaps "How many pens should we have?" is also appropriate, depending on how you interpret "we" (we the staff or we the business?).
For the second question, I guess some customers bring their own cup, mug or flask ("tumbler" is a short glass, eg. for water or even hard spirits such as whisky).
"How much coffee should we serve when the customer brings their own cup? We can't figure out the size (S,M,L)."
(In fact, I would fill the customer's cup with water and pour that into your regular cup sizes. That would tell you how much liquid the customer's cup holds. It might also depend on how many shots of coffee you normally serve.)