i can setup subdomains being the sys admin but how can i let my clients setup a limited number of subdomains other than the prefernces cos that would allow them to use normal domains as well?

lol that makes no sense ok try 2
how can i let my clients setup their own subdomains without them being able to setup nonsubdomains?

thanks

I would suggest pointing them to mydomain.com which will let them point unlimited subdomains to directories on your server and it won't count against your domain usage.