Suppose a test is active for 3 days and total 10000 candidates will take part in it during these 3 days. The server is running 24 hours and the candidates might login at their convenient time during the active period of the test. Since the duration of the test is also fixed, so all these candidates will not remain logged-in and will be able to use the Online Exam system for all 3 days. Any candidate will login and solve the test with one or more logins attempts and will have to leave the system as soon as the total running time of the Online Exam reaches the defined test duration.
In this way, some candidates will log-off in some time and other new candidates will log-in. So the candidates solving the test in these 3 days will be uniformly spread over 3 days and at any point of time there may be about 50-60 candidates logged-in and solving the test. Even if one question paper or several question papers solved by various candidates, the concurrent load on the system is the number of candidates logged-in at one particular instant.
In simple language, the server requires more RAM to be able to run multiple instances of the database server to support multiple logins. Some versions of Operating Systems and Database Server also have limits of maximum RAM that they can use and this puts a check on the maximum concurrent logins it can smoothly sustain. In either case the better the server infrastructure the better concurrent login load can be sustained.
Here it is important to understand that if at any one point of time the number of candidates currently solving the test is not exceeding more than 50 then the server that can handle 50 simultaneous logins of database instances is perfectly ok to conduct a test of 10000 candidates.