Slow cooking chicken breast
Typically with slow cookers, you want the liquid to come up at least 1/3 of the way up the meat.

If you're going to be cooking it for a very long time, if your lid doesn't fit well, or for some other reason you suspect there's going to be quite a bit of evaporation, you may want to add more liquid than that.

There are exceptions to this rule, as the shape of the item in there, if it's something that's going to give off lots of moisture, and the amount of food relative to the size of the slow cooker can throw things off, but 1/3 to 1/2 submerged is a good starting point.