Hi, how people configure their sites is a matter of personal or organisational preference. For instance I have
webprofusion.com.au on the same site and the same certificate. Each of these also has a www. variant, so that would be 4 certs if it was split out, so in this case it’s easier to manage as one managed cert.
You can absolutely have a different managed certificate for each domain if you want to.
Where it gets interesting is when you have a multi-tenant site (for instance an online shop system) with multiple domains mapped to it (one for each client). Usually you don’t want anyone to see that the infrastructure is shared with other companies, especially if they may be quite different products :), so in that case you’d commonly have a separate cert per domain/subdomain.