Start with foot traffic, not budget
The number most people start with is whatever fits the line item, and it's almost always wrong in one direction or the other. A better starting point is expected foot traffic through your booth over the show, not the total attendee count for the event.
A good rule of thumb: plan for 8-15% of total floor traffic to actually stop at your booth, and one pen per stop. That puts a 4,000-attendee show at roughly 320-600 pens for a booth with average draw, more if you're a corner spot or running a demo that pulls people in.
What to do with the leftovers
Undated, un-eventized imprints go back in the supply closet and get handed out at the next event, or dropped into customer-service mailers. Anything printed with the specific show name or date is a one-shot — order closer to your real number if the imprint is that specific, and pad the general-purpose stock instead.
Ordering timeline
Quantity breaks matter more than most people expect at this scale — going from 500 to 1,000 units is often a smaller cost jump than the per-unit savings justify, especially with a union-shop turnaround of about two weeks. Get your art finalized at least three weeks out and you'll have room to reorder if the first estimate runs short.