That will probably come, but there simply isn't enough civs in the game right now to play a 12 civs game. Even thought Civ7 have more civs than any previous iterations on release, when you divide that number by 3 ages, you have the lowest amount of playable civilizations i can remember!
Even for a large, 10 players map,, you'd need to put every existing civilization in every age (exploration being the exception with Shawnees being the 11th civ, but they are not "base game"). 12 players huge is just mathematically impossible. That will probably change as more civilizations are added through DLCs thought. I fully expect larger maps will be available in the future, but the civilization roster needs to grow first.
Well its not impossible to have 12 players with only 10 civs in an age, it just means some would have to be duplicates, (duplicate civs are allowed in multiplayer, no reason it couldn't be auto equipped if # of players was greater than the # of civs that are available that age, based on the DLC the human player has)
I assume they need to (not necessarily in order)
1. make sure their system can handle larger maps
2. see how anything in the game needs to be balanced for larger maps
3. have a solution for allowing duplicate civs when number of players > number of civs available
4 (optional but probably important) Have a decent number of DLC Available so that #3 isn't necessary
On release its 10-11-10 (10-10-10 for everyone)
after March+Crossroads it will be (11-12-12)?
after Rule is out it will be 13-13-13 presumably
That would be the time to introduce a Large Map (12 player 8 Homeland)....
and possibly at the same time or a little bit later a Huge map (18 player 12 Homeland) with the automatic duplicates allowed mechanism [but also for players who just want to play in map with more space ... ie Huge map but only have 12 players]
[As well as probably some map options for 3+ "Lands", more even distribution of civs across the different "Lands", etc.]
Then with those in they are probably set to encourage people to buy up the DLCs that will get it to 20-30 civs per age.