Topic: Determine number of tenants that can be supported on a single server

We have a web application (Saas kind) which is yet to go production however we have a production set up of single app/web server + one DB server.
We have used single DB per tenant approach coupled with apache's SUBURI to enable multi-tenant support for our ROR application.
In essence, I have a root URL in the apache config file and have each tenant as a sub URI, like www.xyz.com/tenant1, www.xyz.com/tenant2, etc.... Each tenant is a rails app having its own DB although the app code and DB schema are replicas of each other.
I am trying to determine the maximum number of tenants that can be supported on a single APP/WEB server using this approach. Please share your thoughts on how to go about doing this.