12 in 10-15 years is right around 1 per year and would result in more LHDs than even the US currently has or ever plans on having. You would also have to group other assets with these LHDs like LPDs, ACVs, fleet escorts, MEUs, helos, crew, maintenance, etc., so it's not just an LHD that you're building. You have to build a whole slew of other things just to make this LHD fully effective. People think China should somehow match numbers with the USN when they constantly forget that the USN CANNOT afford its current numbers and has to massively deficit spend to maintain its current military strength.
Also I don't see China having the need for 12 LHDs. A lot depends on how they configure or use their LHDs, if it operates more as a light carrier or really in an amphibious assault role.
They do have 6 LPDs and a 1:1:1 ratio of LPD:LHD:CV is not out of the question. I think that's the most we will see in 10-15 years, perhaps additional LPDs up to a 2:1:1 ratio.