There's no explicit write up on that specific question. There's two elements in licensing for VMM/VMC:
1. The provisioned virtual appliance resources (CPU/RAM/Disk). What is provisioned or configured dictates the virtual platform sizing outlined in the Virtual APpliance Installation Guides. More CPU/RAM/Disk raises the capacity of the appliance (just like buying a large controller)
2. What the platform is licenses for.
So in your case, with 20 APs, you provision an MC-VA-50 (because 20 won't fit on an MC-VA-10 and we don't provision an MC-VA-20), and then you install 2xMC-VA-10 licenses. This is like buying a 7010 controller that can support 32 APs, but only licensing it for 20 APs (because a 7005/7008 won't fit 20 APs). Exact same thing.