You should not focus on the concurrency rate, rather try to get traffic statistics, such as visits, amount of activity, and so on. If you already have a site running, you can analyze the server logs or any audit trails your application might write. When your site is still offline and you just want to go live, you have to come up with some expectations for traffic.
These numbers will give you an idea what to simulate.
But if I got your question right, you are looking for a way to get the possible max throughput out of your application to know what you can expect of it, rather coming with numbers and trying to prove that you can make the numbers. Right?
This basically calls for a performance test, where you have to set your expectations first:
- what is still acceptable performance (response time limits)
- what are the standard scenarios for the application, what kind of usage patterns do you anticipate
- what kind of data growth can you expect based on the usage patterns
- what is your standard deployment scenario in the beginning, what hardware do you want to us (because you do not know yet, what you need, you have to set something first)
Based on that you can develop you performance scenarios and run it. If you have never done a test before, you should not expect to get an answer to you question rather to find a ton of issues around the application. After fixing that, you will get an answer to your question and get an idea, not a fixed number.
You might also like that article of mine about getting numbers for load testing including some ideas about concurrency: http://blog.xceptance.de/2011/06/07/get-the-right-load-mix-out-of-a-few-numbers/ This is a little ecommerce centric... but might give you still an idea what you might need or what you can do without numbers.