Apple - 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.

Your question doesn't specify which model of MacBook Pro (Mid 2012) you're asking about and the answer may be different depending on model. I summarise this below:

MacBook Pro (15", Mid 2012)

This model has two memory slots capable of taking 204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM memory chips. Officially (i.e. according to Apple) these models only support 8GB of RAM. However, real world tests have proven that they can actually support 16GB of RAM.

See these MacBook Pro: How to remove or install memory for instructions.

MacBook Pro (Retina, 15", Mid 2012)

This model has no memory slots and came with either 8GB or 16GB of onboard memory installed. You cannot upgrade the memory on this model.

MacBook Pro (13", Mid 2012)

This model has two memory slots capable of taking 204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM memory chips. Officially (i.e. according to Apple) these models only support 8GB of RAM. However, real world tests have proven that they can actually support 16GB of RAM.

See these MacBook Pro: How to remove or install memory for instructions.

Hope this helps.