Historically, AMD's biggest dies have had codenames in multiples of 10; Polaris 10, 20, 30, Vega 10, 20 are all the largest dies of those particular architectures/generations.
Polaris 11, 12, and 21 were smaller dies, cut-down from 2304SP of the full Polaris chip to 1024 and 640SP
I don't think Navi 12 will be a larger chip - The halo market is vanishingly small and expensive to operate in. AMD are far more likely to be chasing the mass-market where 90% of sales are with a lower-TDP, cheaper-to-make chip that serves the $200 and lower market, as well as being laptop-friendly. I'm only guessing, but I think that *if* AMD make a larger Navi die, it'll be called Navi 20, or given an new codename altogether.