the limit depends on hosting’s company limit
see
Is there a limit to the number of subdomains?
Each domain name can have up to 500 subdomains. You can also add multiple levels of subdomains, such as http://info.blog.yoursite.com. A subdomain can be up to 255 characters long, but if you have multiple levels in your subdomain, each level can only be 63 characters long. (google)
