The OpenVMS License Management Utility Manual includes a description of the LICENSE command as well.
Software Product Description of a product describes its general capabilities and detailed specifics regarding supporting software and hardware.
License units required for all VAXes and Alphas can be found online at www.compaq.com/products/software/info in the Reference Material tab. A typical entry looks like this:
System Model System Units Layered Product Units AlphaServer 4000 1 CPU 400 1100 2 CPU 500 1100 3 CPU 600 1100 4 CPU 700 1100