Finding the perfect balance between medium and acrylic paint takes some practice. It is also not possible to make a general statement about the mixing ratio, because it depends on the consistency of the acrylic paint as well as on the consistency of the casting medium.
However, we can give you a rule of thumb that always works: Put one part acrylic paint and one part pouring medium into a cup. Mix both for half a minute and then look at the consistency: It should end up similar to liquid honey.