Why we use low numbers for concurrent in MySQL benchmarks or HTTP_LOAD?
It really depends on what you are modeling. For mysql there are a few different definitions for simultaneous users.


Number of database connections, including idles that only take up memory.
Number of running queries.
Number of queries active in the database engine. This is typically ignored.


With a typical web application, you may have a hundred connections open to the database, but only a handful actually running queries at any single time. You can view this number by running SHOW STATUS LIKE 'Threads_running';. Perhaps run it in a loop and see how it changes. Benchmarking the inactive connections is typically a waste as they just use some memory and not much cpu. So if your application only is running x number of concurrent queries at a time, might as well only benchmark x, even if it's a small number.