Is there any restriction of sub-domains ?

You can host unlimited sub-domains. Sub domains are created on a DNS server and they don’t need to be registered with a domain registrar, of course, the original domain would need to be registered before a sub domain could be created. You can create any number of sub-domains with your doamin, for grouping the related information e.g support.cpwebhosting.net, forums.cpwebhosting.net.