How many SSL certificates I must buy in a service oriented system?

I am using Ruby on Rails 3 and I would like to know some information in order to buy SSL certificates.

I have a RoR application at this URL

pjtnam.com

and another RoR application at this URL

users.pjtname.com

If I make an HTTPS request from the ap开发者_如何学Cplication pjtname.com to the application users.pjtname.com like this (in this example I use the Typhoeus gem)

Typhoeus::Request.get(https://users.pjtname.com/sign_in)

and I must receive the response over HTTPS protocoll, how many SSL certificates I must buy?

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UPDATE

How many SSL certificates I must buy if

1. my pjtname.com and users.pjtname.com applications are located on different server?

2. my applications are pjtname1.com and pjtname2.com and they are located on different server?

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If done properly, just one; a wildcard certificate for '*.pjtname.com' and 'pjtname.com' (it's possible to include more than one name in a single certificate).

While you could instead get a certificate for 'pjtname.com' and 'users.pjtname.com', such a certificate would no longer be sufficient when you later decide you need a 'mail.pjtname.com' or a 'chat.pjtname.com' or whatever. The wildcard would cover these future cases.

re: update

1. It doesn't matter if services are located on different servers; just put the certificate on both servers.

2. It's possible to have both 'foo.com' and 'bar.com' in a single certificate, just like you can have both 'pjtname.com' and 'users.pjtname.com'. However, it might more difficult to acquire the foo/bar certificate than the pjtname.com/users.pjtname.com.

Also, these answers are true regardless of platform.