Hi,
I've seen best pratice tips around 10-20 vmdks per LUN. But I don't think those numbers are generally valid because it depends on so many factors that differ in every environment. For example:
What type of disks do you use ( SATA / SAS / SSD )
How many disks?
What RAID level?
What IOPS do the VMs generate? More read or more write?
Is it one vmdk per VM or is the VM splitted into, e.g. C:-vmdk D:-vmdk
etc.
If you want hard numbers: During SAN maintanance I once had 60 system-vmdks (C-partitions and /-partitions) on a single RAID-5 LUN of 11 SAS disks. Had no impact on the user experience, but the read/write latency shown in vSphere was not that nice 
Regards