There are a number of different ways you can manage this. One popular approach is to use the Apartment gem to create what is known as the "multi-tenant" database pattern. See if that term gets you closer to what your are looking for.
One database (or at least one schema) per site is the usual method, so maybe say a little more about why you feel that you need to have the same database for multiple sites. What makes these sites special, or what makes the number of databases constrained?