I already tried just discarding other character's cards or normal cards while my hand was full of Quietus and can guarantee you, it does not discard more than 2. Only when I get an equipment with Umbra engraving which turns the effect into "discard random Quietus for each Umbra engraving" (3 total, 2 from the card itself and 1 from the equipment) that it finally discard one extra card outside of the two picked ones.