One gateway per site may be sufficient - there is no limit on the number of VMs you can add to your connection list.
WAC only connects to one node at a time, so the question of scale really only applies to the number of folks that will be using each WAC gateway concurrently.
In our test automation, we typically have more than 50 simultaneous test cases running in parallel, which is equivalent to more than 50 concurrent users all performing management tasks through a single Windows Admin Center gateway at the same time. Our standard test gateway configuration is a VM with 2 virtual procs, 4 GB of RAM, 100 GB storage (non-SSD). With this configuration, we see an acceptable level of performance and response time through the gateway with no timeouts.