microsoft server 2012 licensing virtualization

Will be setting up a VMware datacenter and I'm trying to decide on licensing options for Microsoft 2012.  All hosts will have 2 sockets.  I understand 1 datacenter license is unlimited VM's per host and 1 standard license is 2 vm's per host.

It's difficult decision because we will not have that many VM's per host (less than 10) so standard will make more sense financially.  I'm just not sure exactly how many VM's will be on each host until we set them up and figure out the resources needed.  So I dont want to to tie the actual license to the host until we figure that out.  Is the license actually electronically attached to the host or is it just figuratively (i.e. 4 hosts with 20 total VM's requires 10 standard licenses regardless if its 6,6,6,2 or 6,6,4,4 or 8,6,4,2)

If it's electronically tied / enforced, how do you 'transfer' between hosts?  We will be purchasing volume licenses not OEM.

Thanks!