Do I have to have an SSL certificate for each web page I want to secure?
No. One SSL certificate covers every page within your domain. If your SSL certificate is issued for www.mydomain.com, then your SSL certificate will work for https://www.mydomain.com/index.htm, https://www.mydomain.com/support/about_us.html, https://www.mydomain.com/books/mysteries/authors.html, and so on.