Single ESXi setup in vSphere is possible, although you won't be able to test/play around with most of the functions of vSphere as those would require 2+ hosts (High Av. etc.)
It is also possible to create a two ESXi vSphere environment without shared storage between the hosts. Shared storage is required by some advanced features of vShpere however you don't have to use them in your lab environment.
Also, if you have an access to a host with at least 16GB of RAM, you could potentially play around with an idea having of two 'nested' ESXi hosts (VMs with ESXi as the OS) running on ESXi on the physical host. Nested ESXi hosts are capable of running VMs of their own ('double nested') with all basic features such virtual networking. Read about it here: Running Nested VMs