how we should know, that how many viewers are using my website? is there any option to view from my webserver?

Depending on your provider, you might have a control panel that will give you this information. Almost all web servers have logs, so you could get the information out of the log. Perhaps an easier way would be to get a web counter for your site, which will give you lots of useful information.

Yes, you can analyse the webserver log files. This gives you a good idea of how many people requested a website, and which one. There are lots of programems out there that do this.

Alternatively, you might look into Google Analytics or somethign similar, perhaps a counter with stats.

It depends on what operating sistem you got your web server, unix/windows. You have a server or you have a site on a paid server ?! If you got your own server, than you might look to see if you got Webalizer, its a program who monitorizes the trafic on your web server (native on Linux). Or try other software like that, you can find lots, google it :)

Goog luck !