Big O(n log n) and Quicksort number of operations

Question

I have an Array with 1,000,000 unsorted elements. I need to calculate expected number of operations that needs to be performed to sort array using Quicksort algorithm in common situations (not the n^2 worst case).

I am not sure how (n log n) is calculated - does it even makes sense to calculate this?

If (n log n) = (n*log(some base)n) what base would be for Quicksort?