If you are running too many VM´s on your datastores in your vSphere environment you can have some problems, this if your SAN is not VAAI compliant and can handle SCSI locking etc.

Alan Renouf has made a blog post about how to get a report about how many VM´s you have on every datastore, I have extended to only report on running VM´s as these are the interesting number..

Get-Datastore | Select Name, @{N="NumVM";E={@($_ | Get-VM | where {$_.PowerState -eq "PoweredOn"}).Count}} | Sort Name | Export-Csv -Path C:\temp\vms-datastore.csv -NoTypeInformation -UseCulture

And the report loooks like this when imported into excel