operation impossible

A riddle from The Riddler on how many different numbers one could at most produce from six initial values and the four basic operations. In other words, how many values could the terms in

a∅(b∅{c∅[d∅(e∅f)]})

could take? (With each ∅ being one of the four operations and a,…,f the initial values or a permutation of these.) A very crude evaluation leads to an upper bound of a million possible values, forgetting that addition and multiplication are commutative, while subtraction and division are anti-commutative. I tried a brute force approach, rather than exploring the tree of possible paths, but could not approach this figure by far, the number of different values still increasing for the largest manageable number of replicas I could try. Reducing the initial values at n=3, I could come closer to 123 with 95 different values and, for n=4, not too far from 1972 with 1687 values.