Rune Sets
13 rune sets of 6 Set Runes each: 4 for the Warrior, 3 for the Sorcerer, 3 for the Hunter and 3 for the Monk. Every set has bonuses at 2, 4 and 6 runes equipped.
How the bonuses show
The rune tooltip lists each bonus of the set with the number of runes it needs in brackets, and the set name with the number of its runes equipped out of 6.
Where Set Runes drop
Runes drop only from boss encounters of level 40 and higher: each cleared one has a 2% chance to drop a rune, 1.5% on mini-boss stages. Only runes the current hero's class can use drop. Set Runes drop only on Inferno: 0.5% of the runes there before the stage's loot bonus and Item Find, which raise it. A Set Rune stays tradable until it is equipped.
Drop chances are the game's rules for offline characters; the game server decides the loot of online characters.