Infinitely many.
4
There are only four different configurations.
Ignoring rotations, there are 3 distinct solutions.
A rectangular prism that is 4 cubes by 2 cubes is made up of 8 cubes.
24 of them
13
4
If you have got enough cubes, as many as you like.
You could try 14!
3
A rectangular prism that is 4 cubes by 2 cubes is made up of 8 cubes.
about 10
25
With no repetition of shapes (symmetry) we have obtained 26 different combinations.
Well, honey, if the height is 4 cubes, that leaves you with 12 cubes to work with for the base. You can arrange those 12 cubes in various ways to form different rectangular prisms. So, technically speaking, there are multiple rectangular prisms you can create with 48 cubes and a height of 4 cubes.
The answer depends on the number. Note that the question does not require the solids to be in the form of cubiods (rectangular prisms).
8 if you want it to be 2X2 of cubes, 27 if you want it to be 3X3.