mid 2012 macbook pro

See here for specs

https://support.apple.com/en-us/HT201165

A lot of users here recommend

http://www.crucial.com/usa/en/apple-memory

You have provided us no info to allow us to specify what memory you need.

Go to crucial.com and use their memory tool to determine which memory is compatible.

You can get OWC/Mac Sale memory here

http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3_160 0MHz_SDRAM

Crucial memory also works, just make sure what you get says it is for a 2012 Mac Pro

I would go with 16 GB or if funds are limited replace one of your sticks with one 16 GB stick