Set two pumpkins together and the eye pairs them. Set three together and the eye moves between them.
That difference sounds like a design-school abstraction and it shows up immediately on a real porch, which is why odd numbers turn up in nearly every arrangement that looks intentional.
What's actually happening
An even group resolves. Two pumpkins become a pair, four become two pairs, and the eye finishes with them quickly because there's nothing left to sort out.
An odd group doesn't resolve. Three pieces can't be split evenly, so the eye keeps moving among them and reads the cluster as one thing rather than as a set of pairs. That extra half-second of attention is the whole effect.
It also reads as gathered rather than placed. Things that grew and were collected tend to come in awkward numbers; things arranged by a person tend to come in matched sets.
Use odd numbers inside groupings
Within any single cluster, aim for three, five, or seven pieces. Three is the workhorse.
Vary the sizes while you're at it. Three pumpkins of identical size read as a set no matter the count; three at clearly different sizes read as a composition.
Use even numbers for framing
The exception, and it's a big one: anything meant to frame the door should be even.
Two corn stalk bundles bracketing an entry, two mums flanking the bottom step, matched groupings on either side of a symmetrical facade. Framing is deliberate by definition, and the pairing is the point.
So a symmetrical porch often has an even number of groupings, each containing an odd number of pieces. That's not a contradiction — it's the two rules doing different jobs at different scales.
Gourds are the exception
Gourds work in twos and threes tucked into seams, and nobody is counting them. Their job is filling gaps rather than forming compositions, so the count matters far less than whether they're touching something.
The rule that governs gourds isn't the number, it's that none of them sits alone in open space.
When to ignore this entirely
If you have four large pumpkins and the porch wants four large pumpkins, use four.
Odd numbering is a tiebreaker rather than a law. It tells you what to do when you're deciding between three and four, and it's a reason to move one piece to another grouping rather than a reason to leave material in the box.
The bigger principle underneath it is variety — different sizes, different heights, different colors. A grouping of five identical pumpkins is still going to look like inventory.