16 core zen, even if its on 7nm, is going to to be very thermally dense and hard to cool without a larger cooler and or lower voltage. Holding back seems like a good idea given how reviews are going to work. But, there is also something else to gain.
AMD is focusing on volume right now. They could take two working 8 core chiplets and make 1 cpu, or, they could make 2 cpus at the lower end where there is volume. You could keep some of the chiplets for your eventual 16 core high end, but, there is not much volume there. If your 12 core part can beat Intel, then going above that is not worth it right now.
AMD would likely be better off waiting until they can meet the mid to low end demand, and launch their 16 core when 7nm has had some time to get refined. A 16 core chip would need to have chiplets that use lower voltage.