Start with the visitor numbers the stand realistically gets, not the total attendance for the trade show. A usable rule of thumb is one bottle per conversation, plus what the stand staff drink themselves over the course of the day. Add a reserve for the last day, which is the day it tends to run out.
The minimum order at Pure Arctic is 120 bottles. From there the price steps down at fixed intervals, at 240, 480, 720 and further up. The steps mean that a small jump in quantity sometimes lowers the price per bottle, so it is worth looking at the step above the first calculation before the order is placed.
A surplus is rarely a problem. Bottles that did not go out at the stand work in reception, at the next customer meeting or in a welcome pack for new employees, and they can be kept for the next occasion. A stand that ran out at eleven, on the other hand, has no reserve to draw on, and that hour does not come back.