By allowing multiple VMs into one datastore, you can run more than 256 virtual machines in a DRS cluster.
As a Host has a limit of 256 scsi IDs that means it can access no more than 255 shared luns. 1 scsi ID is used for local storage.
In a DRS cluster every host should be mapped to the same datastores to ensure the virtual machines can be moved across the cluster and run on any of the hosts.
Now in my life as an VMware PSO architect I haven't seen that much virtual machines that required a single datastore. The primary reason to isolate a VM on a datastore is to provide enough I/O performance. This can be achieved with other solutions that to isolate a vm on a datastore. Having a properly architected storage subsystem is crucial, vSphere can push out an immense number of IOps. With technologies as SIOC and Storage DRS you can make sure the virtual machines receive the I/O performance as needed.