Hari why do we need such a huge number of servers
For the Blomstra platform it's not that we use many large servers, we just add a new one whenever the traffic to our communities (demand) is increasing.
Hari why can't flarum make a static version of discussions which are older than a few months
A static version does not take into consideration any of the information for a logged in user. Like the last posts they've read, which tags they have access to or what posts you did or did not like. Rendering and storing a static html version of your community is possible though and it would boost performance a lot, but whenever someone logs in you'd need to fallback on the non-static version regardless. In addition whenever anything changes (new discussions, new posts, new extensions, design etc) you will need to rebuild those static html versions as well.
Hari so we can decrease the load on the server by delivering a static version to them
Serving a static html version is one way to improve performance and reduce load on the server(s), absolutely! The technical implementation might be harder than coming up with it though, because you need to properly fall back on the non-static version. And you won't be able to provide the security that CSRF provides for instance, unless you work around that.
Hari these days i am quite thinking in go serverless way
Serverless is just another way of using servers, but you will not see how many you really use. If you are worried about having to work with servers, you can also choose for a managed solution (saas).