Nearly everyone under-orders, and the reason is that yard-sign pricing drops steeply with volume, so the difference between ordering enough and ordering too few is usually smaller than people assume.
Campaigns
Plan on signs for supporters who ask, plus a reserve of roughly a third for replacements, high-visibility intersections and the ones that disappear. A campaign that runs out of signs three weeks before the vote has bought advertising and then switched it off at the worst moment.
Contractors and trades
Work out how many jobs run concurrently, multiply by the number of weeks a sign stays on site, and add spares. Most trades cycle the same twenty to fifty signs all year, collecting as they go — the batch is a fixed asset, not a per-job cost.
Events and weddings
Walk the route from the main road to the door and count the decisions a driver has to make. Every turn is a sign; every ambiguous fork is two. Order arrow sets in left, right and straight variants rather than one design flipped, because you will need all three.
Open houses
Four to six signs per property is typical: one at the property, and a chain from the nearest main road. Agents who buy a set of twenty and reuse them across listings spend far less per weekend than those ordering per house.
What volume actually does to price
The per-sign price falls sharply as quantity rises, because setup is fixed and the printing itself is fast. This is why the minimum here is 12 but almost nobody stops there — ask for pricing at two or three quantities on the same quote and pick after you have seen the difference.