Depends on what they are doing... If it's 400 people at a seminar all hitting the same site for a training session at the same time, maybe not... If it's casual use and there is good qos and bandwidth sharing rules, I think it could easily be enough...
I'll put it this way, our municipality, which has maybe 400 employees at computers in various parts of town, has, I believe a 100/100 fiber connection which also serves 911 and some of our web facing portals. I'm unaware of a problem. When I visit my it friends, I think the connection sits with about 30mb of constant traffic.
Someone more knowledgable than me may be able to give you some
Guidance.
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Lots of different usage profiles. I am surprised at both ends of the usage spectrum in my "day job".
If the even has content all the guests will be accessing put a caching server on the captive portal so those elements don't have to be donwloaded repeatedly.