How many brochures do I bring to a trade show?
One approach is to base your estimate on a bottom-to-top basis.
Thinking about where your booth is located and using the trade show promoter's statistics on previous attendance, break down the total time you'll be on the show floor into one-hour increments. Estimate how many visitors are likely to pass by in each hour, and how many will ask for literature (or how much literature you'd like to pass out).
Add your hour-by-hour estimates together, add a fudge factor, and see if the answer seems right to you.
Another technique is to estimate on the low side, then as a backup print a supply of post cards that can be handed out in place of a brochure and also used later as a follow up to the trade show.