You will definitely get 12GB of RAM, but do not expect speeds to be optimal.
Do you need 12GB of RAM? More RAM does not mean better gaming performance, it only means you can run more programs/programs that use more RAM. 8GB for now/16GB to be futureproof is good.
You may want matched RAM pairs to get better performance, and faster RAM up to 3000-3200 MT/s range.
Dual channel only works for two similiar sticks of the same capacity and speed, in this case dual channel may not work, and if it does it will work up to 8GB or 2x of the lowest capacity modulr and at the speed of the slowest module.
RAM operating in single channel may affect performance of CPU+GPU such as the PC you have listed by 0-10%.
Unpaired sticks of RAM may not work at maximum rated speed, but are guaranteed to work at the minimum JEDEC specification of DDR4 2133 C16, where the chips are not running at maximum potential.
You may want a i5 7500/7600 with 8GB RAM than a i5 7400 with 12GB RAM as the i5 7400 has comparatively low clock speed and hence low single thread performance for gaming.
GTX 1060 is fine for 1080p/1440p gaming.