16 GB maximum, in the form of two SO-DIMMs.
Macs can be picky about RAM, so in general, I would suggest getting RAM either from Other World Computing or from Crucial. Crucial "currently does not have any compatible memory updates" for the Late 2012 Mac mini – so that makes the choice of OWC (see link that den.thed posted) an easy one.
You've got one of the Minis where the RAM is really easy to upgrade. There's a large, screw-off access hatch on the bottom of the Mini. Apple provides instructions on how to replace RAM, and OWC has installation videos on their site. (Just take normal anti-static precautions, including not letting feline static generators be with you in a room where you're performing the upgrade.)
Upgrade or install memory in your Mac mini - Apple Support