This question came up in Live Answer Time today and I thought I would post the official word from Hash (link is to store page):
Can A:M use multiple cores? A:M is currently not fully multi-core-optimized. However A:M is able through Netrender (included for free in v16.0 and up) to use multiple cores for rendering. OpenMP is used to utilize multiple cores for several operations. More operations are altered to use multiple cores in each update. Since A:M is a rendering application, it is recommended to use a CPU as fast as possible to decrease rendering time.
So if you have an 8 core system, base AM may not make full use of all 8 cores however if you pick up an additional 4 or 10 netrender nodes for the nominal cost of $10 or $15, you can utilize the other cores as background render nodes.
So those of you contemplating Threadripper/Ryzen/Ryzen 3000 updates may find this useful info.
*EDIT: I should add, most programs are not fully massively-threaded aware yet. I expect that to change over time, but for right now Netrender is the best way to make use of those extra cores.