Voracious Wurm’s ability checks how much life you’ve gained during the turn, not what your life total is compared to what it was when the turn began. For example, if you start the turn at 10 life, gain 6 life during the turn, then lose 6 life later that turn, Voracious Wurm will enter the battlefield with six +1/+1 counters.

In a Two-Headed Giant game, life gained by your teammate isn’t considered, even though it causes your team’s life total to increase.