there are 26 letters in the alphabet. how many cubes will he need to paint once? how many faces on the last cube will be empty

Each cube has 6 sides.

If you have 4 cubes, you have 4 x 6 = 24 sides; too few.

If you have 5 cubes, you have 5 x 6 = 30 sides; this will work.

Putting one letter on each side, you have 30 - 26 = 4 sides without a letter.

And you can just paint those sides black😀