You want to know if additional SSL certificates need to be purchased for Subdomains. For example, your NetCommunity site is www.mysite.org, and you want to use give.mysite.org. Is an additional SSL certificate needed for the new subdomain?
If your NetCommunity site is hosted by Blackbaud, each subdomain requires a unique SSL certificate.
If your NetCommunity website is not hosted by Blackbaud, please contact the appropriate software vendor or IT professional for assistance with this process or issue which is beyond Blackbaud's scope of support.