Can I use a single SSL certificate for all my domains?
Yes! You could use a single SSL for all your domains if you purchased a multi-domain (SAN/UCC) certificate. However, please keep in that you will have to install it manually for each domain (main domain, addon domain, and/or subdomain). The manual installation of an SSL is required because addon and subdomains have their own unique virtual host entry for the Apache configuration. Alias domains share a virtual host entry with the main domain. Each virtual host must have an SSL installed to it for SSL to function.
If you use AutoSSL, you will not be able to use a single SSL for multiple domains. Each domain will have a unique SSL installed for it. However, when using AutoSSL, certificates are automatically requested, generated, and installed, so you do not need to worry about it.
One exception is when there are multiple domains in the same virtual host, for example using a single SSL for alias/parked domains. Subdomains, main domains, and addon domains all create a separate virtual host so each of those needs to have its own SSL.