If you plan to keep the garage fairly cool all the time then a 12K BTU unit should do an OK job in a 400 square foot insulated space. But what it won't be able to do is cool that space quickly if you let the garage, and all the stuff in it, get hot between uses. It may also struggle a bit when you bring in a hot car.
The 12K BTU's is a rating of how many BTU's PER HOUR the unit can move from inside to outside. A garage that has been at 90+ degrees all day is storing a LOT of BTU's in the slab, sheetrock, framing, tools, cars, parts, shelving, etc. Plus, as you're trying to remove all that heat from the garage the heat outside is still working its way into the space. A properly sized a/c system will keep a cool space cool and keep up with the outside heat that is doing its very best to get inside. However, it will struggle mightily to cool an already heated space. It's just the laws of physics and thermodynamics at work.
I'm planning to go with an 18K BTU system for my 480 square foot garage. It's a bit oversized, but inverter systems throttle back the compressor to maintain temp so I believe that it will do a fine job at keeping the space comfy most of the time and still be able to ramp up and knock back the hot air introduced when I open the garage door to move things in and out.
