In short. Between about 20 and 60 for a room where everybody can plausibly meet everybody with no structure beyond a start time. Above 60 you are not running a bigger networking event, you are running a different kind of event, and it will only work if you add structure to replace what the small room did for free.
Organisers are usually pushed towards bigger. Bigger looks more successful, sells more sponsorship and photographs better. It also, past a certain point, makes the event worse at the thing it exists for, and the point arrives earlier than most people expect.
The arithmetic
A useful conversation at a networking event runs somewhere between five and fifteen minutes. Call it ten. A two-hour event, minus arrival, minus any speaking, minus the twenty minutes nobody admits to spending by the food, leaves roughly 70 minutes of conversation. That is seven conversations, and seven is generous.
So any individual meets about seven people, whether there are 30 in the room or 300. What changes with size is not how many people somebody meets, it is the probability that the seven are the right seven.
In a room of 30, seven is a quarter of the room and the odds of missing the one person you needed are low. In a room of 300, seven is two per cent, and without help the evening is a lottery with worse odds than the small one.
The four bands
Under 20. Intimate and high-pressure. Everyone will speak to everyone, so the composition of the guest list is the entire product and a single inappropriate attendee is very visible. Works brilliantly for curated rooms, dinners and peer groups. Fails if two people dominate, so somebody has to be willing to chair.
20 to 60. The sweet spot for open networking, and where most good recurring rooms sit. Big enough that a bad conversation is not the whole evening, small enough that the room feels like one room. Needs no structure beyond a start time, a clear finish and somebody introducing newcomers. If you are starting something, aim here and resist growing out of it.
60 to 150. The awkward band. Too big to be one room, too small to justify the structure that would fix it, and the size at which organisers most often conclude that networking does not work. The room splits into clusters, the clusters are mostly people who already knew each other, and a newcomer can genuinely spend an hour speaking to nobody. If you are here, you have to add structure. See below.
150 and above. A conference crowd, and it needs conference apparatus: tracks, tables, timed formats, facilitation, or a tool that lets people find each other. With that it works well, because the depth of the room is real. Without it, it is the least effective networking format there is, dressed as the most impressive.
Structure is what replaces the small room
At 30, the room does the work. Above about 60, you have to supply it. The options, roughly in order of how much organising they need:
Named tables by topic or sector. Cheap, effective, and it gives people a legitimate reason to approach a group.
A newcomers’ briefing fifteen minutes before the start. Solves the single worst experience at a large event, which is being new in it.
Facilitated introductions. Somebody whose job for the evening is spotting people standing alone and introducing them. Costs you one person and transforms a large room.
Timed formats. Speed networking and similar. They guarantee everybody meets somebody and they are widely disliked, so use them in a slot rather than as the whole evening.
Letting people see who is coming in advance. The structural answer, and the one that scales: if attendees can read the room before they arrive and arrange two conversations, a 300-person event is better than a 30-person one rather than worse, because the depth of the room becomes an advantage instead of a problem. That is what The Gantry does, and it is the reason the product exists in the shape it does.
Capacity is not attendance
Plan the room from your real attendance rate, not your registration count. If you reliably convert registrations at around three quarters, a venue that holds 60 and a list of 80 is a full room, and a list of 60 is a thin one. Get that wrong in either direction and the evening suffers: a half-empty room reads as a failure regardless of who is in it, and an overfull one is too loud to talk in.
How to reduce no-shows covers getting that rate up and, more importantly, knowing what it is.
The underrated variable
Acoustics. A hard-floored, high-ceilinged room of 80 people is louder than a carpeted room of 150, and in a room where people cannot hear each other, the conversations get shorter and more superficial and everybody leaves early without knowing why.
Visit the venue when it is noisy before you book it. It matters more than the catering and nobody checks it.
If you are choosing a number
Start at 40. Grow only when the room is consistently full and you are turning people away, and when you grow past 60, add structure in the same month. Most events that stopped working stopped at the point they grew past the size their format supported.
Related: running a networking event people return to, and how to measure whether a networking event worked.