The certificate is not trusted because it is self-signed wordpress code example

Example: The certificate is not trusted because it is self-signed wordpress

You can get a certificate for each subdomain, a multiple subdomain certificate 
or a wildcard certificate (for *.yoursite.example).

They typically cost quite a bit more than regular certificates though, and 
because you share a single certificate they are typically not the best option 
from a security point of view unless you host an anything.mydomain.example type 
of application where they are the only workable choice.

You also don't need multiple IP addresses if you have a SNI-capable web server.
This said, SNI is only supported in modern browsers (IE6 and below won't work 
with it). Recent versions of Nginx and Apache support SNI transparently (just 
add SSL enabled virtual hosts).