How many oysters do I need?

Count everyone. Yes, everyone.

For easy math, let's say you're hosting 100 people. What we constantly hear is: "I'm expecting 100, but only 50 or 60 of them eat oysters."
No.
Whether your crowd is more or less oyster-enthusiastic is certainly a variable, but you should count every attendee, not just the ones you've decided are likely oyster eaters. Because in any crowd of 100, the guests sort themselves into cohorts:

Uncle Gary (1 per party): This man cannot fathom that we will keep shucking until we reach his nonexistent limit. The kind of person who never says "when" while the waiter grinds Parmesan into a mountain on his salad. If it weren't for common decency and a faint air of shame, Uncle Gary would consume the entire lot. Budget him 25+.
The oyster lovers (about 5 guests): Garys with restraint. Count on 12–15 each. They're indulging, as they should.
The casual oyster eaters (about 15 guests): The type who occasionally puts in an appetizer order of oysters but doesn't push the limits. 4–6 each.
The oyster curious (about 25 guests): Here for the party, happy to participate. 2–3 each.
The partygoers (about 35 guests): Food? Why not. They'll try one or two, or none. If oyster shooters or torching are in play, count on more.
The oyster avoidant (the rest): They recoil at the sight of our sea treasures. We can only hope they eventually see the light.
Add it all up and a crowd of 100 lands between 230 and 300 oysters. That's why 250–300 is our sweet spot for this size. Heavy on oyster lovers, or blessed with a few Uncle Garys? Go 350.

Do I need to buy oysters for guests who won't eat them?

This is the question behind the question, so let me answer it directly: you count everyone, but you're not buying 3 oysters for each non-oyster eater. The per-person average already accounts for them. The people eating zero are subsidizing Uncle Gary's 25.
The mistake is discounting twice: first by ordering for "the 60 who eat oysters," then by applying a per-person figure that was already built for a mixed crowd. Do that, and you'll run out in the first hour.

The variables nobody warns you about

The weather. Our anecdotal studies have conclusively shown that oyster consumption ramps up with UV index and drops with cloud cover and rain. An exceptionally beautiful day deserves a bump. We acknowledge that this is nearly impossible to plan around when ordering for a party weeks or months in advance, but we figured we'd tell you anyway.
Your own psychology. Would running out at the ninety-minute mark horrify you, or was that the plan all along? Neither answer is wrong, but you should know which host you are before you pick a number.

Small parties generally eat more per person

So if 100 people need 250–300, then 20 people need about 55, correct?
No. Or, at the very least: it depends.
Small parties are intimate, and everyone has permission to be Uncle Gary. You're among friends, after all, and a live shucker is an experience meant to be taken advantage of. For headcounts of 30 or under, plan on 4–5 oysters per person: 80–100 for a party of 20, 100–150 for 30.

Big parties eat fewer per person

At the other end, scale down. Unless you're staffing 3–4 shuckers for a party of 250, there's only so much shucking that can happen before the party moves into its less-oystery phase. For 250 guests, 450–600 oysters is the right zone.

Weddings are their own animal

Weddings run on different math: 1.75–2.5 oysters per guest.
(Unless, of course, your wedding is exceptionally large or small. Size matters)
Two reasons. First, the raw bar usually works the cocktail hour, a tighter window than a backyard party that sprawls all afternoon, and your guests know dinner is coming. Second, and this is the part most vendors won't say out loud: at a wedding, running out is the goal. Nobody wants to end their wedding night thinking about what to do with a cooler full of shellfish. A raw bar that's picked clean by the last call of cocktail hour did its job perfectly

You know your crowd better than we do

Everything above is more of an art than a science, and every number in this article is an average. Averages assume a distribution: some guests will have 8 oysters, and some (most) will have 0–2. That's already baked in.
But you've known these people your whole life. We'll know them for about two hours. If you read all of this and think "my crowd will blow through that" or "there's no way we get past 200," trust that instinct. Outliers exist, and the host is the one who spots them. Our recommendations are the baseline for an average crowd; we'd only move the number up or down if you're genuinely confident your people eat much more or less than most. If you are, say so, and we'll build the order around your read instead of our table.

An honest word about budget

We’re very aware that parties are expensive and everyone is looking for places to save. So here's my honest take: the shellfish order is not the place.
You've already committed to paying for the bar and service, which is the bulk purchase. The one outcome that makes a party feel under-provisioned, and reflects poorly on us as much as the host, is the bar running dry 45 minutes in.
When we recommend a quantity, the only thing on our mind is providing the best raw bar possible for your event, not seeing how many oysters we can stick you with.

So, how many oysters do you need?

It depends. But now you know what it depends on, and the table above will land you within a dozen either way.
Still not sure? Tell us your headcount and your crowd, and we'll give you a real number. That's the part we're actually good at.