Migrating Exchange 2007 to Office 365, Go-Daddy Hosted Website, How many SSL's do I need?

My setup is Windows 2008 server, with locally hosted Exchange 2007.  I'm trying to migrate to Office 365 Business Essentials on a cutover migration.  To do this, I've learned I need to enable Outlook Anywhere.  To do that I need an SSL.  Right now our email addresses are firstname.lastname@webdomain.org  Webdomain is registered through GoDaddy who also hosts our web site.  We have a MX record that directs our email to mail.webdomain.org and an A record for host mail that points to our static IP.  We currently have no SSL certificate anywhere that I can find.    I don't know how many SSL's I need, and I don't know how to make sure I'm requesting them correctly to make sure I'm covered for the migration.  I don't even know if I will need one for any longer than it takes to migrate to Office 365.  Can someone help point me in the right direction?