I have always operated on the "rule of thumb" that a datastore should not contain more than between 8 - 12 VMs, and so I size my luns/datastores accordingly, taking into consideration the I/O nature of the vms, as well as (of course) the size of vm disks
Too many active vm disks can create a contention scenario on the storage array -
But you need to know your workloads, and to know your storage system capabilities, really.