Two, one at the head of each syllable, and neither doubles. English almost never doubles a j at all. The letter is a late arrival in the alphabet, split off from i in the sixteenth century, and it never picked up the doubling habit that b, d, m and the rest have. Jujube is a good place to notice that, because a doubled j would have to be modelled on something, and there is no ordinary English word to copy.