Historically, VMware provided a free license key for standalone hosts. It restricted management via vCenter Server and limited physical RAM configuration per host, though it allowed unlimited cores per CPU in version 5.5.
In the wake of Broadcom's acquisition of VMware and subsequent licensing changes, many organizations are considering alternatives. If you are looking at ESXi 5.5 due to cost constraints, you should evaluate these modern, powerful, and free/libre open-source alternatives: vmware esxi 55 license key 14k