I have the following domains, all inside one single VPS (one IP address):
www.example.com
classmates.com
john.classmates.com
mary.classmates.com
blog.classmates.com
If I need https for the following domains:
https://www.example.com
https://classmates.com
https://john.classmates.com
https://mary.classmates.com
What's the minimum number of SSL certificates required?
It could be done with a single SSL certificate. Specifically, what you want to do is to provide 'Subject Alt Names' in the certificate.
Take a look at Google's certificate for example, which has the following Subject Alt Names. In other words, they have one certificate for all of those domains.
DNS Name: *.google.com
DNS Name: *.android.com
DNS Name: *.appengine.google.com
DNS Name: *.cloud.google.com
DNS Name: *.google-analytics.com
DNS Name: *.google.ca
DNS Name: *.google.cl
DNS Name: *.google.co.in
DNS Name: *.google.co.jp
DNS Name: *.google.co.uk
DNS Name: *.google.com.ar
DNS Name: *.google.com.au
DNS Name: *.google.com.br
DNS Name: *.google.com.co
DNS Name: *.google.com.mx
DNS Name: *.google.com.tr
DNS Name: *.google.com.vn
DNS Name: *.google.de
DNS Name: *.google.es
DNS Name: *.google.fr
DNS Name: *.google.hu
DNS Name: *.google.it
DNS Name: *.google.nl
DNS Name: *.google.pl
DNS Name: *.google.pt
DNS Name: *.googleapis.cn
DNS Name: *.googlecommerce.com
DNS Name: *.gstatic.com
DNS Name: *.urchin.com
DNS Name: *.url.google.com
DNS Name: *.youtube-nocookie.com
DNS Name: *.youtube.com
DNS Name: *.ytimg.com
DNS Name: android.com
DNS Name: g.co
DNS Name: goo.gl
DNS Name: google-analytics.com
DNS Name: google.com
DNS Name: googlecommerce.com
DNS Name: urchin.com
DNS Name: youtu.be
DNS Name: youtube.com
However, it becomes more difficult for signing authorities to be sure about common ownership of multiple domains.
It will likely be far more economical to buy several certificates. Check with your certificate signing authorities. There are actually many options available.
Be aware that the prices are dramatically different whether you want one domain, one domain with subs or several domains. So its really all about how much you want to spend vs. how much convenience from reduced certificate management you want.
Also, some signing authorities might charge more if those hosts reside on more than one IP address rather being hosted from the same IP.
Two:
