As with the engine figures, there are usually prominent 3 figures
1997cc

Engine Displacement
V6, V8, V12

Number of Cylinders in V-configuration
12, 16, 20 valve - The number of valve on cylinders.
No...there is no standard engine size, similarly, even say for a 2.0-litre there are different displacement like 1997cc, 1998cc, 1994cc, 1954cc just to name a few example. It all depend on the manufacturer. Usually to make thing simple we round it off.

You only mentioned V6, V8, V12 but there are other kind of configuration. The most common is S4 (aka in-line 4). In-line 6 are also quite common (BMW 325, 330i, skyline, etc). Also there are flat-configuration F4, F6 like those found in Subaru and Porsche.
Valve. Usually, there are 3/4 valves per cylinder (2 in, 2 out), so 16V will mean a total of 16 valve (4 for each cylinder).