What is the maximum RAM size for MacBook Pro Mid 2012?
The Mid 2012 MacBook Pro can support up to 16GB of RAM by using 2 8GB kits. Both the Retina and non-Retina models (mid 2012) support 16GB of RAM.

Per EveryMac.com, you can have 

I have personally used the Kingston KVR16LS/8 modules many times with great results.



Since, you said that you already upgraded your drive to an SSD, I assume you know how to remove the bottom cover to access the memory.