Lets start with viability which means basically having thousand of different builds you can make. Blizz stated over 2 trillion possible builds! what? how....lets discuss.
Lets say 25 skills per char, 5 skill runes per skill, gives you 125 diff skill options per char. You'll have 3 diff armor sets you use, mf/gf, leveling and pvp. Now back to skills you can have what 7 usable so 125/7 = 18 diff builds but x that by the 125 = 2250 = the possible number of skill builds.
Now thats a lotta builds.
So #of skills times #of skill runes divide by # of skills you can use then multiply by #of skills x runes number
Lets say 25 skills per char, 5 skill runes per skill, gives you 125 diff skill options per char. You'll have 3 diff armor sets you use, mf/gf, leveling and pvp. Now back to skills you can have what 7 usable so 125/7 = 18 diff builds but x that by the 125 = 2250 = the possible number of skill builds.
Now thats a lotta builds.
So #of skills times #of skill runes divide by # of skills you can use then multiply by #of skills x runes number