We are switching web hosts, and I have been asked to find out how many concurrent users the Magento-based websites have in order to estimate the appropriate hardware.
How can I find this information out?
The webserver is lightspeed (like apache) and it is PHP-based.
Other information that may (or may not) be helpful is that the sites are currently hosted on a shared hosting solution, so I don't think I can install any monitoring software.
I have noticed that Magento has a built in report that may be similar to this... Admin -> Customers -> Online Customers. But I have a feeling this report isn't really what the new web host is looking for.
Should this question be posted in another Stack Exchange site?
Sign up for Google Analytics at http://www.google.com/analytics/. They will provide you with a tracking code to paste into the html of your site. Insert that into your header template. It may take half a day or longer for the stats to accumulate.
Then take a look at your peak hourly stats. Click visitors then switch the graph to hourly and take a look at the hour with the max visits. That will help you approximate the amount of traffic your new host will need to accomodate. Also take a look at pageviews since that's also an important metric. Visits and pageviews are not the same as concurrent users but it should put you in the right direction.
Are you moving to a dedicated or at least dedicated resource vps? If you're currently on a shared host, it's really hard to get a true sense for what type of hardware you are going to need (I'm assuming the reason you are switching is because of performance problems). I'd suggest starting with a basic dedicated server and then either going up or down from there based on your results.
You could try the m1.small instance at Amazon EC2 which will cost you about $70/month and you'll be able to host multiple sites on there. Of course you'll have to manage the server yourself.