You should also remember to subtract 2gb for Windows to use for general computer operation. So right now you are using about the max amount of ram you would want to use. If the new DLC eats up more than about 1gb of ram, you should look at upgrading.

Cities will run even if it requires just a bit more ram that you have in your computer, but will be stuttery and slow, as it will need to keep reading the hard drive to swap assets when your ram is full.

Before I even started to play C:S and all the DLCs, I upgraded my system.
I doubled my RAM to the MB's max of 32GB (2*8GB to 4*8GB), replaced my video card & replaced the CPU (FX-4100 @3.6GHz with 4 threads to a FX-8350 @4.0GHz wirh 8 threads).
That was still almost CAD$500 cheaper than buying a totally new AM4 DDR4 based system.

16 GB = 16384 MB.

Like RED onion pointed out, the Operating System will never give all RAM to Cities. 13.2 is pretty much the maximum the game will get with 16 physical.

I suggest looking at virtual memory usage. As long as it stays under 20, you should be ok. Or 18-19 if you want to be very safe. After 20, you'll see slow operation and eventually the game crashes.