Permutations

Permutations are groupings of elements by order.

The mathematical way to calculate the number of permutations is called a factorial.

Triads have 6 permutations:

R35, R53, 3R5, 35R, 5R3 and 53R.

For triads you have 3 chord tones and therefore three options for your 1st choice, two options for your 2nd choice, and one option for your 3rd choice.
3! = 3*2*1 = 6 (this is called 3 factorial)

Sevenths Chords have 24 permutations:

R357,     R375,    R537,    R573,     R735,     R753
3R57,     3R75,    35R7,    357R,     37R5,     375R
5R37,     5R73,    53R7,    537R,     57R3,     573R
7R35,     7R53,    73R5,    735R,     75R3,     753R

For seventh chords have 4 chord tones and therefore four options for your 1st choice, three options for your 2nd choice, two options for your 3rd choice, and one option as your 4th choice.
4! = 4*3*2*1 = 24 (this is called 4 factorial)