6k to 12k users connected for around 1.5 to 2 hrs. It is kind of an online examination.

You're asking the wrong questions; or at least, providing the wrong information; and of the wrong people.

It's simple to demonstrate that a single commodity box with an average amount of memory (say 4GB to 8GB) with suitable server software can sustain 12000 concurrent connections with ease. If they are doing nothing or very little.

Whether those 12000 users would be serviced in a timely manner if they all attempted to connect at the exact same moment is a different matter; but that's normally not a concern as it is rare for 12000 people to coordinate their actions in that way.

Of course, for an exam, you might stipulate that they must start at exactly the same time. Even then, you would normally tell them that they must be logged on a least 5 minutes before the start time, and give them a 15 or 20 minute window before that during which to get logged in etc.

Assuming that you do require them all to see that first page of the questions at "exactly the same time", then the concern is, can your server deliver that first page to 12000 users within some specified period of time. Assuming sufficient bandwidth and ignoring net latency, that equates to can your server/software deliver that first page to all 12000 within that specified period.

Given the first page will be the same (or substantially the same) for all of them, that means a 'static page' delivery. Even if the first page is customised -- with the user name/ID etc.; as you have this information for 5 minutes before the off; those customisations can be done before the off, thus avoiding active content generation at that peak period.

After that, each user will move through the pages of the exam at different speeds, which will have a natural tendency to spread the load out.

So the headline number you need to look at is how much content (kbytes) needs to be delivered to each user over that 1.5 or 2 hour period; and how much resource -- CPU; DB; etc. -- does your software need to generate that content.

Once you have those figures -- easily measured from your existing or test system setup -- then you have what you need to perform your capacity planning for the numbers involved.

Bottom line: asking random strangers on the net to guess how to do your job; when only you have the information required to do it; is likely to lead you in completely the wrong direction.