It reads like performance testing/engineering is not your core discipline. I would recommend hiring someone to either run this effort or assist you with it. Performance testing is a specialized development practice with specific requirement sets, tool expertise and analytical methods. It takes quite a while to become effective in the discipline even in the best case conditions.
In short, you begin with your load profile. You progress to definitions of the business process in your load profile. You then select a tool that can exercise the interfaces appropriately. You will need to set a defined initial condition for your testing efforts. You will need to set specific, objective measures to determine system performance related to your requirements. Here's a document which can provide some insight as a benchmark on the level of effort often required, http://www.tpc.org/tpcc/spec/tpcc_current.pdf
Something which disturbs me greatly is your use case of "at the same time," which is a practical impossibility for systems where the user agent is not synchronized to a clock tick. Users can be close, concurrent within a defined window, but true simultaneity is exceedingly rare.