When you get the free license, you have to register how many physical machines it will run on, but not aware of any limit. I think there might be a two physical CPU limit per physical box but don't actually see that specified, and I have not installed 5.5 yet... (so someone will likely correct me if I am mistaken).
I did find this on a blog: TinkerTry IT @ home | Best parts of VMware’s ESXi 5.5 free hypervisor rely on vCenter, which isn’t…
... all the good stuff is only gotten from the vSphere Web Client. This is a rude awaking for many who may not have tried ESXi before. Examples of this good stuff include VMs with greater than 2TB drives, mapping USB 3.0 ports to VMs, better GPU handling, and vSphere Flash Read Cache. These compelling features are locked to hardware version 10 VMs, which cannot be managed from the vSphere Client, and cannot be run on ESXi 5.1.
So the biggest limitations appear to be you can't tie into vcenter / web client and are thus restricted to using virtual hardware version 9 (and below) VMs and their limitations. That said, it does sound like you can manage many settings of virtual hardware 10 VMs with Vmware Workstation 10.
So you either want to make your you keep with vm hardware 9 machines, or at least have a version of Vmware Workstation 10 to connection to ESXi 5.5 hosts.