I have seen domains such as www.uk.domain.mobi. Is there no restriction in terms of what is allowed after www?
Can I call a subdomain anything I like and is there no dot-separated word limit?
No. At the very least after the second level domain (
See this for more infos.
Simply put (in addition to restrictions mentioned in other answer):
Yes, once in control of the DNS records you can create subdomains, sub-subdomains, sub-sub-subdomains, etc.