[andrew.lee]

So we count everyone who currently has a tab open on our page. In our experience, 1 simultaneous user on average equates to roughly 1000 unique visitors per month on the "average" website. This number is often higher for forums, and higher still for really tight-knit forums. On *our* website, you'll notice we tend to average about 15-20 people at a time, and our website has pretty typical stats. If you have 1000 visitors per day I think it unlikely you'll have hundreds at any given time -- unless you are holding scheduled events of some kind. Most people tend to sleep, have jobs, eat meals, browse other websites, etc : )

I don't know the details of how vBulletin estimates usage. I think they just guess based on how many requests have occurred recently. They tend to guess quite high in our experience.

We're actually running a pretty complicated setup with multiple different servers each serving up a different piece of our service. The real-time webserver portion is built on top of a server called Jetty which is specifically designed for this type of usage.

As I said, we have free trials on all our plans, so you can try it out and see how many simultaneous users you have. If it's too many you wont pay a cent.