Can I Use A Single SSL Certificate for Multiple Subdomains
To secure all subdomains of your domain, you will need to get a wildcard certificate. At Namecheap, you can purchase PositiveSSL Wildcard, EssentialSSL Wildcard, and PremiumSSL Wildcard.
The procedure of activation and installation of a wildcard certificate is the same as for ordinary ones. You can find the instructions here.In this article, you can check more details about how the domain name should be specified for a wildcard certificate and which subdomains will be secured.