VMware ESXi imposes some limits on virtual machines depending on your license level and the version of ESXi you're using. This page details what the most important limits are and provides a link to VMware's PDF documentation for further details.

Version VCPUs / License Memory SCSI devices Disk size Release date
4.0 8 / Standard 255GB 15 2TB (less 512B) 2009-05-21
4.1 8 / Standard 255GB 15 2TB (less 512B) 2010-07-13
5.0 32 / Enterprise 1TB 15 2TB (less 512B) 2011-08-24
5.1 64 / Enterprise Plus 1TB 15 2TB (less 512B) 2012-09-11
5.5 64 / Enterprise Plus 1TB 15 62TB 2013-09-22

Something else to keep in mind is that prior to the 11.10.310 release all Enterprise releases were 32-bit, which supports a maximum of 8 VCPUs at the OS level. You can see the supported number of CPUs like this:

grep NR_CPUS /boot/config-`uname -r`

On our 64-bit VM, we support a maximum of 256 VCPUs at the OS level (which is far more than ESXi currently allows):

grep NR_CPUS /boot/config-`uname -r`