If you mean the lightmap scale, then 16 is the default value which is usually a pretty safe fallback.
The smaller number you make, the higher your shadow detail will be, but at the cost of compile time and fizesize. An entire map of 16 can have almost 20mb of lightmap data, if you make it all 8, that's 40mb, 4 is 80mb, 2 is 160mb!! Of course this works in reverse too, so making faces that have no need for detailed shadows higher values will reduce compile time and filesize.
I'm not sure where you got the idea of make it all scale 2, or 6 or 12, but that's very bad practice unless you know what your doing. For most maps, you should never need to change it from 16.
Also it's best to keep it a power of 2 (2, 4, 8, 16, 32, 64) because texture sizes etc etc binary space partition etc etc.