Cake Size: Since you're using 8" cake layers, the base of your cake will be 8" in diameter.
Ghost Can Dimensions: You should find the dimensions of the Ghost can you want to replicate and use those to scale your cake. For example, if the Ghost can is 6 inches in diameter and 8 inches tall, then you'll want your cake to be close to these dimensions.
Volume Calculation: Calculate the volume of the Ghost can and then the volume of an 8" round cake layer to estimate how many layers you'll need. Ensure the total volume of the cake layers matches the can's volume.
The volume of a cylindrical can (V) = π * r^2 * h, where r is the radius and h is the height.
The volume of an 8" round cake layer = π * (4")^2 * (height of one layer).