Hi there, blog writer here. I can lay out the evidence as to why MOST DF-31A brigades have twelve launchers.
First thing we can do is look at the infrastructure of each brigade base. DF-31 brigade garrisons are built to a very particular standard. You can spot it if you look at each location. The launcher garages are all long, usually around 25 meter drive-through garages (meaning, the garage is open at both ends). This is the only system in the PLARF that uses this drive-through configuration. Now originally, most of the brigade bases only have a single set of garages for support vehicles and launchers. So the TOE was six launchers, six launch control vehicles, six command vehicles, eighteen cable vans, and eighteen support equipment vans. Over the past five to six years we've been seeing brigade bases build more infrastructure to host more launchers and more support equipment. I've attached before and after shots of Yuxi, a confirmed DF-31A garrison, so you can see what I am talking about. The drive-through garages are the things in the upper right, in the middle of the garages.
Second piece of evidence is the latest numbers from US gov's China Military Power Report 2019. The report gives us the total number of launchers in each range class, so the total number of deployed ICBM launchers is cited to be 90. Because we know the ORBAT for the PLARF, we can break that down based on each brigade. The silo forces are the easiest to count - they have three brigades, each with six silos, so subtract 18 launchers, giving us a remaining total of 72 launchers. All these remaining launchers are DF-31 or DF-31A brigades (the deployment location for the DF-41 is still under construction, and is thus not yet deployed). Because we accurately measure the infrastructure at each brigade base, we can estimate how that breaks down, as the PLARF has exactly 72 places to deploy the DF-31 and its variants. Five of the brigades have twleve launchers, and two have only six, which is why I say that DF-31 brigades USUALLY have twelve launchers.
Finally, we can count brigade numbers my examining where they build the vehicle TELs. I have an article coming up that delves into this in more detail, and I don't want to snipe myself on it on this forum, but basically because the production locations ship completed TELs out in brigade sized batches, we can see a completed brigade set before they deploy, which serves as additional confirmation that brigades have 12 launchers per brigade.
As a note, Hans actually does say in his count that the DF-31 and its variants have six or twelve launchers per brigade, he's just counting less launchers than I am. He counts one DF-31 brigade with six launchers, two DF-31A brigades with twelve launchers, and two more DF-31AG brigades each with twelve launchers, for a total of five brigades. There's some disputes over the two brigades that I'll get into at a later date.