I've been wondering that exact question for some time in relation to our EVA4400 and ESXi 3.5. Older VMware documentation suggests as few as 8, but I recently quizzed our local support person who suggests it's more like 12 to 20.
Poses a dilema - creating losts of smaller LUN's and leaving some headroom for snapshots etc increases stranded capactiy.
I'm hoping someone will provide a more scientific answer.
The reason you have not found a definitive answer is becaue it depends on a number of factors - what is the disk i/o load from the VM, how your host is configured, how the storage is configured - but according to the configuration maximum the number of hosts that can share a VMFS volume while running virtual machines against that volume is 32 - http://www.vmware.com/pdf/vi3_35/esx_3/r35u2/vi3_35_25_u2_config_max.pdf
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The maximum number is written in the above document.
There isn't a "best" number. It depends of your storage, the number of disks (that compose the LUN), the type of disks, the type of RAID, the type of SAN (FC/iSCSI)...
Anyway you can find in different discussion that this number can vary between 8 and 20 VM.
My suggestion is to limit to 10 VM per LUN.
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I/O is key, so you need to know your disk subsystem and what load each vm brings with it. Memory, CPU and network access are just as important to baseline on each VM-canadiate you have. To give you a real world senario, I have two ESX 3.5 boxes, Both are Dual - Quad core, 28GB Ram with (6) 15K-FC 300 GB drives carved into RAID 5 LUNS on SAN. My TEST VM's (20) total run fine in this configuration as the I/O is Spiky as Developers and Programmers and test users run in this enviornment. My Production VM's (6) are all I can run on the second ESX 3.5 server and PRD LUN so that I don't bottleneck.
Hopefully this gives you a little insight