Thirteen
The short answer, related to the spades answer there, is that there was a built in prejudice towards them that has lingered and outlasted it's usefulness. It's not a surprising bias if you consider that when the cards first appeared, those who used swords weren't very nice people. Tales of King Arthur aside, the only ones using swords were noblemen in power over everyone else—or trying to get into power over everyone else. Swords were used hurt, kill, subjugate, threaten, and behead. And not much else. So seeing swords in your future usually meant bad things. A sword battle was bloody and ugly if you had a sword. If you didn't, you were gonna die.Compared to the other suits, the suit of swords by far has the most negative cards. Why is this?
So, swords had this "death" reputation rather like spades—and besides, it's an easier read if one suit is the "bad luck" suit. When it turns up, you know bad things are coming. If it doesn't, then all might be good. Anyway, Waite went along with this when he created his deck, and as the vast majority of tarot decks are clones of Waite, the negative towards swords continued, even if it made no sense.
Fortunately, many have decried this bias (Crowley most notably, he loved swords) and created more fairness in viewing the sword suit, less bias, and more equality in general. You just have to get the right decks and books.