Choose the number based on portions. You will want to, at minimum, match the total number to your number of attendees.
For example, I have 38 attendees and would like to have Hot Coffee as well as Water. I would choose 38 Hot Coffee and 38 Water if I want every attendee to get some of both. If I only want to have enough so that some people have Coffee and others have Water, then I may choose 20 Hot Coffee and 18 Waters.
