Hey there, I currently play games regularly with 12 Civilizations under these conditions:
I have changed the size of my Standard map size to be larger since I am uncertain if Large and Huge map types are slightly buggy right now, and play *exclusively* on the Fractal map type since I have been customizing it to suit the demands of 12 players purely through the global map parameters in order to ensure that spacing between Civs is adequate and reliable. All other map types break down and are not playable with the changes I have made unfortunately. Ignoring that obvious flaw, I do play with this in multiplayer and besides the highly desired fact that the spacing is fine, human players (so far) have always spawned on the home continent and not Distant Lands which I have seen some edits fall into the problem of.
I emphasize that you MUST manually select the 12 Civilizations in the lobby (you will need to duplicate 2 because there are only 10 Antiquity era Civilizations released right now) otherwise the AI will not spawn in at all. Random leaders appears to be completely fine for me to this point.
I have shared the edits I have made in the attached folder and where to place them in the Readme. I tend to like things closer than most, but you can yourself edit the grid width and height if you feel you want more or less space via the maps.xml file.
Enjoy!