it depends on how much cake you have. start by adding just a few tablespoons at a time and mix. Keep adding untill you get the consistency you want. I like to make it so that it is the consistency of chocolatecip cookie dough but other people like it mushier or stiffer. I think it is a trial and error kind of thing not an exact recipe. HTH.
Have you seen the tutorial link on this site? It does not tell how much liquid to put, but it has a good explanation on how to make them.
http://cakecentral.com/article67-How-To-Make-Cake-Balls.html
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