Each Hosting package allows a different amount of sub-domains. Started packages allow 10 sub-domains; Business packages allow 50; and Professional packages allow 100. When creating a new Sub-domain, ensure that you are not exceeding your quota for the package you are using.
To create a subdomain on your 123-reg hosting package, please do the following:
Login to your 123-reg control panel.
Scroll down to the web hosting section.
Select the relevant domain name from the drop down box and click on the Manage button.
Click on the Sub Domains option in the Web Tools section.
In the Domain Name field enter the part that will come before .yourdomainname.co.uk and in the Directory field enter the directory that the subdomain will map to. Press the Create button to complete the process.