128-bit bus = four 32-bit memory controllers
192-bit bus = six 32-bit memory controllers
256-bit bus = eight 32-bit memory controllers
384-bit bus = twelve 32-bit memory controllers
Micron, Samsung, SK Hynix,
et al manufacture GDDR6 with 8 or 16 Gb densities, and in the case of GDDR6X, from Micron, just 8 Gb at the moment. There are no other densities available.
GDDR6/6X operates so that a single DRAM module is connected to a single controller, in 32-bit mode, or two modules per controller, each in 16-bit mode. So the memory configurations possible are:
128-bit bus (4 or 8 modules)
8Gb modules = 4 or 8 GB
16Gb modules = 8 GB or 16 GB
192-bit bus (6 or 12 modules)
8Gb modules = 6 or 12 GB
16Gb modules = 12 GB or 24 GB
256-bit bus (8 or 16 modules)
8Gb modules = 8 or 16 GB
16Gb modules = 16 or 32 GB
384-bit bus (12 or 24 modules)
8Gb modules = 12 or 24 GB
16Gb modules = 24 or 48 GB
AIB vendors will want to use as few DRAM modules as they can, to keep costs down. The higher density modules
are roughly twice the price of the lower ones, when purchased from retailers, although they'll be cheaper than this when bought direct. AMD have gone with 16Gb for their RX 6800 lineup, Nvidia is stuck with 8Gb for GDDR6X.
There's a small but not insignificant price difference between module speeds too, roughly $5 or so, although it varies as according to supply and demand. AMD were obviously happy to absorb this by choosing the fastest, highest density GDDR6 modules for their 6800 series, but now the consumer is going to expect this to be the norm for all future products.
I still think Nvidia's original plan for the 3060 was for it to have 6 GB of 14Gbps 8Gb GDDR6, but once they got word of what AMD were planning with their new products, they switched to 16Gb modules. That way, when AMD releases their mid-range/budget RDNA 2 cards, with 128-bit memory buses, they'll have less memory than the 3060 and thus offer Nvidia a marketing advantage (unless AMD are going with a 192-bit bus too).