How much memory does ESXi require for itself?

Solution 1:

Memory overhead is going to hinge on a lot of things. Luckily, VMWare has a nice little table to let you know what to expect:
http://pubs.vmware.com/vsp40_e/resmgmt/wwhelp/wwhimpl/common/html/wwhelp.htm#href=r_overhead_memory_on_virtual_machines.html&single=true

Also see this
http://pubs.vmware.com/vsp40_e/resmgmt/wwhelp/wwhimpl/common/html/wwhelp.htm#href=c_understanding_memory_overhead.html#1_10_9_9_7_1&single=true

Solution 2:

In practice, based on the few that I run in production, the ESXi core uses around 200 or so mb. Personally, I try to keep around a gig free of non-reserved ram per server (200 reserved for ESXi, 256 mb for Linux guests, 512 to 1024 mb for Windows guests depending on version and services)...