You get one TS, one CS and one snack credit per person on the reservation times the number of nights of the reservation.
The credits can be used any time from the time you check in and get your Resort IDs until 11:59pm of the day you check out of the resort.
If you had four people and arrived on the 18th and checked out on the 23rd you would have had five nights (18, 19, 20, 21, and 22) and would start with 20 credits for each category.