There's no single, simple answer to that question. How large are your VMDKs going to need to be? How many VMs total? How active I/O wise are these VMs? ESXi v5 with VMFS v5 removes the 2TB LUN size limit and is more space efficient than VMFS-v3.
In general terms, more, smaller volumes are better than one MEGA volume. Keeping the number of IO active VMs down on each volume is also a good idea.
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