How many Magic Cubes?
Ruby, 275 bytes



n=gets.to_i;p (2.upto(n**(1/3r)).map{|i|(1..n).to_a.permutation(i**3).map{|x|c=eval'x'+".each_slice(#{i}).to_a"*2;[c,c.map(&amp;:transpose),c.transpose.map(&amp;:transpose)].map{|q|q.map{|a|a.map{|b|b.inject(:+)}}.flatten.uniq.length}.inject(:+)==3?1:0}}+[0]).flatten.inject(:+)/48+n


Try it online!

In theory, this works. In practice, since it creates every permutation of the list of possible numbers, it'll take hopelessly large amounts of time and memory for any input more than 13 or so. I haven't taken the time to test it past 12, but it seems no 2×2×2 magic cubes exist for that range, so all it really does is tell you how many 1×1×1 cubes you can make.

I "golfed" it a bit (really just made it hard to read for minimal size reduction returns). I'm sure the size can be reduced quite a bit, though getting it under 100 characters is a pretty tall order.

Ruby 2.4 added .sum, but TIO doesn't support it yet, so this has a few .inject(:+)s.

I'll add a non-golfed version + explanation if requested.