Basically you would need one for your config site and others for each client site that requires ssl.
SSL authenticates the address so you would need at least one for every domain name that will handle secured SSL traffic. For example
https://secure.buystuff.com would require a different cert than secure.rentstuff.com, if you are using subdomains you can secure everything unter that domain with a wild card cert which covers *.buy.com so secure.buy.com and
www.buy.com could use the same cert but these are more expensive.
A bit corporate but this page may have some answers
http://www.verisign.com/ssl/ssl-info...rks/index.html
Edit: looks like I was beaten to it whilst typing :)