multiple organizations in one CRM deployment, SSL Issue

Hi:

My Scenario:
I have Dynamics CRM installed locally.

on this deployment, I have multiple organizations configured.

and my organizations urls are:

localhost/.../orgName1

localhost/.../orgName2

localhost/.../orgName3

localhost/.../orgName4

I am required to get SSL Certificate that will handle request at my serer.

My question is:

am I required to get One SSL Certificate?

or I am needed as many certificates as many organizations I have at my server?

please suggest?

as I will be adding organizations later and already existing/running SSL will be effected?