If you put any number of -1/-1 counters on more than one creature at once, Hapatra's last ability triggers once for each of those creatures.
2017-04-18 • wotc
If you put enough -1/-1 counters on Hapatra so that its toughness is 0 or less, its last ability triggers.
2017-04-18 • wotc
If a creature with wither or infect deals damage to a creature, the controller of the creature with wither or infect puts that many -1/-1 counters on the second creature.