A good rule is to fill your bundt pans to no more than 2/3 of their capacity. This way the batter won't overflow and the cake will bake well. With some batters, you can get away with 3/4 of the capacity, but that will be more trial and error as it will depend on the recipe. What happens if you fill the pans too much is that the cake will take too long to bake fully drying out the edges and center in the process. And it might overflow.