Answer: Hello your question is missing some details but I will provide a general solution based on the scope of the problem and you can plugin the missing value 
answer = Volume of rectangular prism box / volume of cube 
Explanation:
To determine the number of Dice that will fit in the rectangular prism box
First : calculate the volume of the cube box ( dice ) 
volume of a Cube box : V = L^3 where L = side length 
next : calculate the volume of the rectangular prism box
volume of rectangular prism box = L * b * h 
L= length , b = width , h = height 
final step : Divide the volume of the rectangular prism box by the volume of the cube box ( dice ) 
= ( L * b * h ) / ( L^3 )