Hi currently I have 1 server so I setup dns as ns1.mydomain.com, ns2.mydomain.com ip address pointed to server1. So, now I just bought another server. Can I setup ns2.mydomain.com pointed to 2nd server. Also how to setup more servers ie. 10, 20 server because I can only set up to 6 dns in one domain name.

Yes if you have cpanel you can setup all the dns you want I think via Dns Clustering. I am pretty sure you can use that.

Ok. I know it is in the cpanel but what I should put as main domain name on 2nd server? I have buy cpanel license for 2nd server right? So, if 1st server is using mydomain.com and ns1.mydomain.com pointing to 1st server so what should I do on 2nd server? ok, I set ns2.mydomain.com and what is main domain name there? Should I put mydomain.com also?

Im not totally sure but I think you can just use the same domain since it is like a mirror dns. I am really not for sure though but should work.

I think its possiible. SOme domain control panel do provide service of puttin of more than 2 nameservers.

You should be able to set it up as a mirror as ns2. This has always worked for me!